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3000 BC, Under the Great Rain of Fire

by Francesco Defferrari

I think the first years after the Great Rain of Fire would make a very interesting (and very deadly sub-campaign in which the PCs would have a hard time to simply survive...
The main inspiration of this "timeline of the cataclysm" are the dreadful events after the Chicxulub asteroid impact that destroyed the dinosaurs, events that also similarly occurred during other mass extinctions in Earth's history.

In Mystara's official product I think the Great Rain of Fire was never fully detailed or explained.. the name led me to think it could have been a multiple asteroids impact, yet official products also state that "Powerful technological devices in Blackmoor exploded, shifting the axis of the Known World in an event later called the Great Rain of Fire". But if it was so, why a "rain of fire"? maybe ejected materials from the explosions?
The Age of Blackmoor by James Mishler explains the cataclysm as a Serpentine plot that caused global madness and the attack from a fleet of colonial warships. 
Maybe the advanced technomagical civilization of Blackmoor was able to use asteroids as weapons, but anyway I think that this timeline could apply to any kind of impacts or explosions powerful enough to shift a planet axis..

And the Night became Day.

Time 0, Blackmoor city.
It's the middle of the night and light appear in the silent sky, brighter than the sun. Most of the city is asleep, but the few that see the light are immediately blinded by it and haven't the time to ask anyone what is happening.
Time 0 and 1 moment, Blackmoor city.
The bright sky roars as if ten thousands dragons are about to descend on Blackmoor and most of the citizens are deafeaned by the terrible noise. Everyone awakes in sudden terror. 
Time 0 and 2 moments, Blackmoor city.
The city is obliterated in an immense whirlwind of fire and debris. The 25 millions inhabitants of the greatest urban area in Mystara die instantly. The extremely violent and unexpected cataclysm kills even the greatest part of the mightest clerics and wizards. Just a few individuals can witness the destruction inside flying force-fields, shocked and powerless..
Time 0 and 5 moments, Blackmoor city crater.
The few, terrified survivors can now see an immense cloud of fire that from the former city expands into the countryside as far as the not blinded eyes can see, pushed by a roaring red wind that set farms, forests and orchards on fire. 
The earth below is shaking like a beaten carpet, the mountains are crumbling like sand castles and rivers of lava pour from the ground, while Blackmoor bay is boiling. 
Time 0 and 10 moments, Blackmoor city crater.
Sea level drops like in an emptying cup and to the far east a great wave is rising. The earth still shakes in shock and fires burns everywhere around the dark crater, covered by a huge cloud of black dust. Night is reclaiming the sky now, but it's a night full of bright fallen stars, the debris from the Blackmoor impact (or explosion) that are falling all over Skothar. 

Five hours of Fire

The night that wasn't night, Blackmoor city crater.
For several hours after the event, while the mega-tsunami wave and the cloud of fire ride away and the massive earthquakes begin to subside in the region of the crater, an unexpected consequence hit the few survivors above ground. 
Citation: “For several hours following the Chicxulub impact, the entire Earth was bathed with intense infrared radiation from ballistically reentering ejecta. The global heat pulse would have killed unsheltered organisms directly and ignited fires at places where adequate fuel was available. Sheltering underground, within natural cavities, or in water would have been a necessary but not always sufficient condition for survival.”
I take for granted that such a radiation shut down wizardry and clerical magic in all the planet for at least some hours. All the wizards and the clerics still alive above the Blackmoor crater are immediately incinerated when their force-fields dissipate, except for the very few who teleported away in time. 
Every natural material that wasn't already burned before it is now, and the land is a raging inferno for some time before turning into a desolate desert of burning coals and ashes, under a red, deadly sky that kills any (still) living creature exposed to the light. 

So, how PCs in Blackmoor city could ever survive all this? They can only if they were deep underground at the time of the impact/explosion (or if they are among the very few powerful and smart enough described above). Being underground is not, however, in any way a guarantee of survival. The ground is shaken by massive earthquakes for miles even underground and any structure not sturdy enough will be crushed. The PCs will have probably to flee through falling hallways and rooms, asking themselves what's happening outside. All that will obviously work better if the PCs do not know Blackmoor history or at least they do not knew before that they were in Blackmoor city on the eve of the apocalypse. 
Attempts to save people or retrieve items left above the ground are almost certainly doomed. The DM should warn PCs approaching an exit that terrible noises and intense heat come from the above. PCs able to go outside with enough magical protection to withstand fire and massive impacts have to return underground before the radiation or be immediately killed by it (No saving throws). In that case, the DM should warn PCs that the destruction is not over yet and something even more terrible is about to happen, giving them a small chance of survival... But because the radiation should happen just ten seconds or half a minute after the impact/explosion, PCs probably will not have the time to reach an exit before it... and during it the intense heat coming from above should be enough to discourage anyone..

The true adventure however begins the day after, when the PCs will emerge from underground in a burned, unrecognizable wasteland, with almost nothing left alive, frequent earthquakes, poisonous clouds, lightnings and acid rains sweeping the land, intense heat from global warming and almost zero visibility due to dark clouds blocking the light of the sun. A terrible environment that in a few months will evolve in a burned wasteland rapidly freezing in a dark, nuclear ice age that will last for years...

The question of PCs level is not so easy... low level PCs would be better because they will not have plenty of magical aid and therefore they will have to use wit and intelligent solutions to survive... yet they also will have low HP so probably would need some leniency and aid from the DM...
Mid level PCs will be more resilient but could be tempted to use teleport quickly to escape (even if the situations isn't exactly easy in other parts of the world either, as we'll see later). It would be better, I think, if the DM discourage teleports pointing out that any familiar place the PCs would go to will not be there anymore, or will probably be engulfed in a storm of fire...
The same and more goes for High level PCs, that could use planar travel, time travel, powerful divinations, creation of food and several protections... the DM could decide that magic works erratically at best or that some spells do not work at all, thus limiting the danger that PCs could bypass the adventure entirely...

The Dark Summer

From one day to three months after the event, former Blackmoor kingdom.
The days after

When the PCs will at last be able to go outside, only when the heat of the above fires will cease, they will find a burned wasteland, with burning coals and ashes everywhere, no traces left of the great city that was there before. There aren't even ruins to search, just a miles width, still burning crater covered with poisonous smoke. We should assume that the PCs will emerge outside or at the edge of it, because nothing would have survived right in the middle, not even deep underground. 
The three immediate threats now are the still burning fires everywhere, the poisonous clouds and the frequent seismic activity, including bursts of lava and geysers.
The air is still so hot that it's hard to breath and light is decreasing as the clouds of gas and debris gather in the sky above, dimmer than during the darker storm even in the middle of the day. 
The DM should stress out that's better to leave the area of the city as quickly as possible, also because the return wave of the tsunami is now coming from Blackmoor bay, high and visible to the north east. 
The wave will soon sweep all the former area of the city and fill the crater, drowning everyone still standing there and raising huge and highly dangerous clouds of hot vapor visible for miles. 
Some PCs could think to search the former city area for highly magical objects, the only things that could have survived such an immense destruction. These objects, however, will be now buried several feet under the ground, at best, or crushed by the immense force of the impact/explosion. 
The DM should make finding one of them highly difficult and dangerous. 
Assuming the PCs will run away from the crater, they will face in the next three days several difficulties, among permanent conditions and random events.

The permanent conditions are:
- Spells, either clerical, druidic or wizardry, have a 70% chance of failure due to the intense radiation in the atmosphere. Some, like teleports, divinations and creations should not work at all or have a even higher chance of failure, or life will be too easy for the PCs. The DM could also decide that the impact/explosion has caused a entire week without magic, as in WotI. 
- Due to the clouds, visibility is under 500 feet (150 meters) during the day and pitch black at night, like in dense fog. PCs can only avoid many random threats (see below) by hearing them coming, not seeing them. Magical aid will probably be of little help.
- Orientation is really hard because compasses don't work anymore, mountains and hills have crumbled and changed shape all around the area, there is no vegetation at all and the direction of the sun is always unrecognizable in the dim light. Even expert travelers who know the area very well will rapidly get lost, unless the PCs find some clever way to mark their direction. 
- Breathing is always difficult due to gasses and intense heat in the air. PCs will sweat and suffer from fatigue and sickness, with associated penalties. 
- There is almost no Water left at all in the burned wasteland, except for the acidic and raging sea, which PCs should obviously avoid. Digging in the ground there is a small (5%) chance each hour to find pockets of dark waters where streams, wells or ponds used to be, or also some small surviving piece of the former blackmoorian aqueduct. There is a high chance (60%) that these waters are poisonous and will make the drinking PC sick, dehydrating him even more. 
- There is almost nothing left alive around Blackmoor city, not even a leaf of grass or a small insect. That also means there is no food. Digging in the ground or searching in the small pockets of water there is a small chance (3% for each hour of searching) to find little insects, hardly enough to sustain grown men or women for a long time. If the PCs don't have food they will suffer hunger for days, until they are sufficiently far away from Blackmoor city. There is however a way to find abundant food: the sea is now full of dead fish floating around. The problem is that the sea will also be full of tsunami waves for days, but clever PCs could find a way. 

The random events are (5% change each every hour):
- Chasm open in the ground
- Local earthquakes
- Bursts of Lava
- Geysers of poisonous gas
- Meteors
- Violent and sudden floods of acid rains
- Moving hot clouds
- Tsunami waves (only near to the coast but up to ten miles inland)

As already said almost nothing is still alive near to the event, yet some encounters are possible in this area (small chance, 5-10% in three days):
- Random monster or animal that somehow managed to survive, because it was underground or underwater. It's probably mad with fear and hurt, so it could attack the PCs on sight, unless there is a druid of another PC able to calm it.. in such a case, the monster/animal could become an ally.
- Robot or golem that survived the event.. It could be damaged and hostile too, unless a PC can disable or control it. 
- Other human or humanoid creature, or a group of adventurers that has the exact same story of the PCs (1d10 people). 

The first days after the event in the former Blackmoor city area should be a really hard time for the PCs, they could despair to find something or someone alive and even think that all the world has been utterly destroyed. Attempts to contact the Immortals should probably fail, as planar travel and teleport. Only after three or four days, if the PCs have managed to distance themselves from the crater, they should begin to find some little sign of hope, like small patch of plants, uncontaminated waters and small animals still alive. 
From the fourth day onward and for the three months of the dark, hot summer the random events and permanent conditions described above will continue, while daylight will decrease day by day, until visibility will be only 30 feet (9 meters) even at midday. The PCs will discover, probably with grief and terror, that in such conditions photosynthesis is becoming impossible, and therefore all the patches of plants left will quickly die out completely. 
Also during this time the first sign of the Wasting could appear among survivors, but I don't know if someone already developed rules for it?
Permanent conditions will not improve in this time, but the random events will change: chasms, earthquakes, bursts of lava, geysers, hot clouds and meteors will gradually become just a 5% a day and then a week probability, tsunami waves will cease, but acid rains will increase to a 15% chance every hour and lightnings storms will begin with a 10% chance each day. 
The dark summer in former Blackmoor area also has these special events that the DM could choose to use or not:
The Wizards: some powerful blackmoorian wizards that were away from the city during the event will manage to return via teleport in the area searching for items, friends or relatives. All of them will be in a state of deep anguish, but probably they will ask the PCs for help. They could also give them some news about other parts of Mystara, where the destruction has been huge but life somehow survives (for details, wait for the next chapters about Oltec, Tangor, Elves and Aquarendi).
The Elven fleet: Aladrian, an elven sky-captain from Evergrun, has traveled through great danger to the Blackmoor area with seven elven sky-ships searching for revenge. Elvenhome has been hardly it by the event like the rest of the world and he considers blackmoorian responsible. He came here planning to wage a vendetta war and will be disappointed to find all the humans already dead. He will be initially hostile to the PCs and will try to capture them, but if they are able to talk some reason into him they could point out that there is no proof whatsoever that humans caused the event. Aladrian could then become a precious ally to begin a search for survivors in all norther Skothar. At some point, he will want to return to Evergrun and the PCs will have to choose if to go with him or disembark somewhere. It's useful to point out that traveling by sky-ships in these times could even be more dangerous than walking, even if the ships have partial (but not full!) magical protections against lightnings, impacts, acid rains and meteors. (A variant of Aladrian could be a survived colonial blackmoorian sky-ship mentioned by James Mishler in his history, the main difference being that at some point the captain will want to return to his home planet...)
The Fire people: with Mystara in such extreme climatic conditions creatures from the elemental plane of Fire will start invading the area to establish a dominion. They have no interest whatsoever to help humans or other mystaran creatures, but rather want to exploit the event to transform Mystara in a suitable environment, that is very similar to their home plane. They are numerous and probably will try to kill the PCs immediately, so some help from the two previous special events could be essential to defeat them.
The Survivors: the PCs could begin to encounter small (10-100 people) groups of humans or other humanoid races that somehow survived the event (probably inside some sturdy structure away from the impact/explosion). Aiding and gathering them involves a great set of problems (feeding, healing and keeping peace and discipline among the groups). 

After playing the three months of The Dark Summer, PCs still in the area of former Blackmoor will have to survive the next season, The Black Rain.

The Black Rain
Four to six months after the event, former Blackmoor and Thonia

Acid rains become more and more frequent, until rain is always present, at various intensities.
The rain doesn't hurt much, but after days it creates minor burnings on the skin (-2 hp for all the season), increasing the chance of infections (5% each day) and exacerbating the damages of the Wasting. 
Visibility is 30 feet (9 meters) only on the brightest days, but normally is pitch black. To travel by sky-ship has become impossible, and on the ground, while there are no more fires nor bursts of lava, flooding an lightnings storms are common (10% change each every hour). 
Temperature drops constantly, of 5°C degrees a week, from the 35-45°C (95-113°F) of the three months before, so is around 25°C (77°F) at the middle of the fourth month from the event, 15°C (59°F) at the end, 5°C (41°F) by the middle of the fifth month, -5°C (23°F) by the end, to stabilize at -15°C (5°F) by the middle of the sixth month, on the eve of winter.. It should quickly become obvious to the PCs that's not a normal summer-fall drop, but the beginning of an unnatural cooling that will only worsen once winter comes. For the moment, the acid rains don't easily becomes snow, but there are some severe snowstorms at the end of the fifth month.
For the PCs and the people that are probably with them now, all this means to freeze, and no food in sight. It should became pretty obvious that going south is the only way to survive. 
But to do that, they'll have to cross former Thonia...

Leaving the former kingdom of Blackmoor PCs should begin to find people, animals, plants and still standing (even if badly damaged) towns and houses. Yet plant will die out completely in the course of this fall, due to the lack of light, and all animals will migrate south, as any intelligent survivors will do to escape from the rains and the increasing cold. 
While the area nearest to Blackmoor was mostly a scorched flat wasteland, going south it will become very common to see bodies of people, animals and monsters killed by the initial event, by the following consequences, by random violence among survivors or by the wasting. All that probably will not improve morale among PCs and survivors.
Moreover, many survivors will still have dangerous weapons of the Blackmoor age, as rifles, bombs, grenades, plasma cannons and much more... These weapons however are working more and more erratically, for the effects of the radiation and also because Immortals are starting to tamper with them for good reasons, so they are even more dangerous and prone to disruptive explosions. 

Encounters (increased chances, 15% each day):
Random monster or monsters that somehow managed to survive. Still probably mad with fear and hurt, so it could attack the PCs on sight, unless there is a druid of another PC able to calm it/them.
- Robot or golem that survived the event.. It could be damaged and hostile too, unless a PC can disable or control it. 
- Other human or humanoid creature, or a group of adventurers that has the exact same story of the PCs (1d10 people). 
- Larger group of scattered survivors (humans, elves, dwarves or other races) going south.
- Pack of animals going south, not really dangerous but frightened, so they could crush the group that the PCs are leading (or following). 
- Local lords still in control of their people and area, mostly with nasty attitudes and paranoia, but some could be persuaded to join the PCs group or aid them.
- Random undeads, that can walk around during the day. 
- Elementals (mostly water), spirits, drawn by the intense rain and the general magic disruption. 
- Survivors with signs of the wasting that the PCs could choose to aid, with likely opposition from their companions/followers. 

The Black Rain period through former Blackmoor and northern Thonia area also has these special events that the DM could choose to use or not:
- Fanatic and frightened groups of humans are leading purification wars against survivors infected with the wasting or suspected followers of death cults. PCs will have a hard job stopping them from killing and burning groups of innocents. 
Undeads are growing in numbers, created by followers of Thanatos and other dark immortals, who are exploiting the hard times to create more chaos and despair. Simply kill the undeads isn't a permanent solution until the hideouts of the dark clerics are found and destroyed. 
Millenarian cults of any kind are spreading among survivors, some preaching about improbable heavens “where people will be safe” in a random direction, others inciting to violence against minority groups. PCs could try to save people from an improvised prophet or have to fight such a cult growing among their group.
Water creatures are invading the area from the elemental plane of Water, defeating the Fire creatures from the previous season. They too aren't much in the mood of bargaining with humans, but clever PCs could at least establish a sort of truce with them. 

If the PCs can survive the Black Rain and lead survivors to the south they will have to face The Grim Winter.

The Grim Winter
From the first winter to the tenth year after the event, central Skothar.

The winter that comes after the Black Rain will not end in the continent of Skothar for the following ten years. During this time, many survivors of the initial event will die for the cold, hunger or the wasting. Some will also be killed by other survivors. 
The DM could want to end the adventure here, after the PCs are able to find some permanent shelter for a large group of survivors. Yet could be more interesting if the PCs can aid the community of survivors several times during this period, maybe jumping through the years thanks to a time travel device, or to the aid of a sympathetic Immortal. 
The community could later become the beginning of a modern nation in Skothar. 

The first winter after the event will begin with temperatures rising to -5°C (23°F) and severe snowstorms that will last for days and cover everything with 200 cm (6.56 feet) of snow. The snowstorms will be followed by days with temperatures between -20 and -40°C (-4 to -31°F) with low point of -50°C (-67°F), and then by more snowstorms. Cycles of snow and intense cold will continue for ten years, and therefore central Skothar will always be covered with a layer of snow and ice at least 2 meters thick.
The planet will still also be covered by a layer of clouds, daylight will always be very dim at best, and no plant or tree will survive the long winter. Living in the wild will become even more impossible, and the only scarce resources of food will be underground.
But obviously all the surviving creatures of this area of the world will seek shelter underground, competing for the scarce resources among themselves and with the original inhabitants of the place (like garls, earth creatures, any sort of fungus creatures, beastmen, worms, insects, rats and other small mammals, lizards, toads, salamanders and dragons).
Underground the temperature is much higher than outside, usually from 0 to 10°C (32 to 50°F) or even higher near to source of lava or underground volcanoes, but there is always the danger of seismic activity, rock slides, geysers, gas and lava. 
The best course of action for a community of survivors will be to find a big, warm cavern in which to farm the only things that can grow in these conditions, that are mushrooms, and in which keep animals for milk, eggs and meat. The problem will be to defend the cavern from monsters, undeads, envying neighbors and having to make difficult choices like to accept or not to accept more survivors seeking shelter, or to forge or not dangerous alliances.
Also, the wasting will not stop in these years, creating more panic and fear in the community.
Many races of survivors could be present in central Skothar around this times:
Neathar humans that will eventually found the modern nations of Thonia and Nentsun, Oltec humans that will become the Jennites, Gnomes and Dwarves that will found Thorin, Rakastas, Gyerians, Pegataurs, Centaurs and Fairy folks, Gatormen and Frogmen, Giants, Beastmen, Dragons and maybe also Elves, Halflings and Lupins who came from Davania and Brun during the blackmoorian age. 
Possible events for these ten years are:
Religion war. The community is divided by religion issues that could develop into a full scale war, with obvious disastrous results. The DM could choose two Immortals with very different philosophies, like Nyx and Ixion or Rathanos and Tarastia.. the losing side could end up exiled out in the cold.. 
Undead cult. With such prohibitive living conditions in the outside world and the wasting disease, many could be tempted to seek the help of Immortals like Nyx and Hel, who can promise salvation in undeath. The PCs would have to choose if embrace or fight those religions. 
. Many groups of survivors will be left roaming the underworld, displaced by war, cold or local disasters. Some of them could band together and wage war on the PCs community, that has warmth and food. 
Desperate survivors. Rag-tag groups of survivors could end up on the cavern that host the PCs community.. these people, desperate for food and shelter, will beg hospitality and many in the community will not be too happy to aid them. If refused entrance, people with nothing left to lose will probably choose violence to the point of suicide, meaning that they will assault the community to the last man and woman, without morale checks. 
- Giant worms. They travel under the earth, and many could pop up in the middle of the caver that the community chose. The downside is that the PCs will have to fight them, the upside is that they could be a source of food and clever PCs could also find a way to breed and train them. 
Dictatorship. In such difficult conditions is almost certain that at some point a messianic leader will rise to dominate the community with unrealistic promises and extensive use of violence against the opposition. That scenario could also be linked to the Religion war and/or the Undead cult. 
The Wasting. The disease will continue to reap victims among the survivors, prompting many kinds of magical researches to find a cure, possible persecutions of the people infected, newborns abandoned in the wild with the probable opposition of the parents, and many other problems. Also, objects or places with a high content of radiance could worsen the disease from time to time. 
Dragons. Them too will seek shelter under the earth. And they will not suffer lowly creatures to encroach in their caverns. PCs should probably try to appease them rather that going into war with such powerful creatures, who will have already plenty of minions. 
Fire and Frost salamanders. With ice and snow above the ground and fire underground, this area of Mystara could well become in these years a battleground between these two outsiders, who are not on friendly terms. To avoid being crushed in the struggle, PCs could play one side against the other or try to ally themselves with the winning one.
Hivebrood. It seems these guys are numerous in Skothar, so a group of them coming to the PCs cavern is likely, and it's unlikely that PC's will be able to negotiate with them.

So if the PCs will succeed in aiding their community of survivors live through the ten years long winter, they will live to face The Deluge!

The Deluge
The spring after ten years of winter, central Skothar

The radiance ice age that kept this entire area of Mystara in a ten years long grim winter will end with a shower of rain that will last three months. With temperature increasing to 15-20°C (59-68°F) on the surface, snow and glaciers will rapidly melt and soon all the land except for the high hills and mountains will be covered by water.
The first consequence is that the cavern hosting the community that PCs are protecting will soon be flooded and the people will have to abandon it. Obviously leading a big group of people complete with food, equipments, objects and animals to safety will not be an easy task. Clever PCs could also find an alternative in sealing the cavern from the waters to survive the three months of deluge (providing the community has enough food to do this).
Even remaining underground the community will still have to face many perils, mostly collapses, mud slides and any kind of creatures tunneling into the cavern to escape the waters, from giant worms to dragons and other intelligent creatures.
Above the earth it will become soon mandatory to build rafts to move the community in the flooded land, with the dangers of flood waves, icebergs, sea creatures washed here, floating debris.
The community could also encounter some creatures that will thrive in an environment of abundant water and rain, particularly cloud giants, blue dragons and water elementals. Such creatures could become precious allies under the circumstances.
Visibility on the outside will slowly improve during these three months, but the sky will always remains covered and dimmed by a deep layer of gray clouds. 
The community will probably wander on raft until the PCs could find dry ground. Before that, the DM could set an encounter with a ship:
- The Thonian ship. These guys are arrogant and will probably try to force the PCs and their people to follow them, unless they strongly object. 
The Fairy folk ship. This crew includes centaurs, fauns, gnomes, gremlins, sidhes and other creatures that haven't much experience in manning a ship. They will accept and give help if the PCs are not too threatening.
The Dwarven ship. Them too don't know much about ships. Wary of strangers, but willing to forge a temporary alliance. 
The Pirate ship. A melting pot of races that banded together to engage in raiding. More interested in stealing than in murder or taking slaves, but still quite dangerous. 
Once the group reaches dry ground they will probably encounter other refugees: any race present on Skothar (thonians, other neathars, proto-jennites, dwarves, giants, rakastas, dragons, froglins, gatormen, beastmen, fairy folks or others) could be present, and their attitude seeing more people seeking shelter in “their” dry land will probably vary from very cautious to outright hostile.
If the PCs are able to save themselves and the refugees from the Deluge they'll live to finally see The Dawn...

The Dawn
The summer after the Deluge, eleven years after the event.

When the rain stop, the sun return at last. It's a wonderful sign of hope for all the survivors. During the summer, temperature will slowly become normal for the season and vegetation will start to grow, only mushrooms and ferns at first, then grass, flowers, plants and trees.
From hideouts all over the land animals and intelligent creatures will go outside, to live in a new land under the sun. And to conquer it. 
In these years Skothar could have several human and not human cultures, like thonians, other neathar cultures, proto-jennites, proto-ethengarians, proto-antalians, dwarves and gnomes, rakastas, fairy folks, centaurs and pegataurs, gyerians, froglins, gatormen, giants, dragons and some groups of elves, lupins, werecreatures and halflings could be present too. 
Outsiders that took advantage of the extreme conditions of the previous ten years could be present too, like any kind of elemental creatures and demons. 
Undead also will be common, as probably several human cultures resorted to death cults to survive.
All this people and groups will try to carve their own realm in the pristine land, and the PCs will have some work to do, forging alliances and fighting enemies, to establish the seed of a future, powerful nation of Skothar.
By this time most blackmoor tech device will be destroyed, lost, not functioning or infected with dangerous radiance, so any struggle will be a stone age war, with some random tech still around
The DM could allow PCs to follow the developing of the new nation till modern times. Indeed this whole campaign could also be appropriate for a PC following the Dynast Path to achieve immortality in the Sphere of Time.

And that's would be all for the first scenario, the cataclysm as viewed by survivors in former Blackmoor and central Skothar. But how the rest of Mystara experienced it? Let's move to Tangor city...

And the Night became Day
The first minutes after the event, Tangor city

The Tangor capital isn't hit directly by the impact/explosion, so it will not be completely and immediately destroyed as happens to Blackmoor city. In the middle of the night, the citizens who are awake will still see a bright light encompassing almost all the northern half of the sky, then everyone will awake under the deafening sound of a thousand thunders. 
The tangorians will have several minutes to come out of their homes as the sound ceases, to look toward the bright red light in the north with puzzlement and anxiety, maybe authorities, wizards and clerics will try to frantically investigate the mysterious event, but they'll hardly have the time to discover anything, as Tangor city will be hit but what later will be known as 

The Dread Fire Night
The night after the event, Tangor city

As the sky becomes more and more brightly red the radiation hit the city as the rest of the planet. Most of the vegetation in and outside the urban area is incinerated, as are animals and people. Only citizens who were inside buildings or under deep foliage are spared the horrible death, yet they have to experience a dreadful night in the city as the capital is hit by a apparently endless succession of earthquakes, meteors, fires and tsunami waves from the Tangor bay. The ill-fated who have to escape crumbling houses are doomed, incinerated in the outside by the red radiation. Clerics and Wizards cannot help anyone, as their spell don't work at all. Only people hidden in secure buildings, the very lucky or very clever survive through the dread night. 
And the Day didn't come.
When the murdering red radiation finally stop and most of the earthquakes and meteors cease the survivors will come out of their hideouts finding an appalling scenario. The city has been destroyed, 90% of the original population lay burned in the streets, the air is thick with burning smokes from the nearby forests still on fire and burned people and animals wander around wailing and crying. 
And so begins the...

Time of the hot dark Death
The six months after the event Tangor city area

Permanent conditions of the first three months of this season are similar to those in Blackmoor written above under The Dark Summer: each day light get dimmer, even at midday, magic has a 70% chance of failure, breathing is difficult due to the intense heat and gasses, yet in this area of the world water, animals and plants have not disappeared entirely. Even if vast stretches of woods are completely burned there is still surviving vegetation in the underbrush, and water has only a 20% chance to be poisonous. Random seismic and meteoric events of the Dark Summer apply, but only with a 5% change each every day.
During this season it's not difficult at all to find food in the Tangor area: in the first days there is plenty of dead animals around, and dead fish in the sea, rivers and lakes, and later, as plants start dying for lack of sunlight, survivors can find dead animals even more easily. It should be clear however that this pattern is not encouraging at all for future survival, so the first and foremost task of this season should be to save as much food and farm animals as possible. Trying to grow anything will eventually fail, all the plants will inevitably die as photosynthesis becomes more and more difficult.
The Togoro (Tangor's Great King) is among the survivors, and will immediately ask PCs and any other skilled individuals to help his people. The tasks to perform are numerous and all massives: find and gather survivors in the city and surrounding areas, create shelters, remove debris, fight random monsters and hurt, mad animals, gather food and water, stop looting and violence, prevent more deaths, heal the wounded, bury or burn the dead. 
Special events of the Black Summer could be used too if the DM wants, and the PCs could also receive the task to investigate on what's happening in the world. Eventually three months will pass, the intense heat will subside a little and the rains will begin, as in The Black Rain season in Blackmoor, but with less intensity. For the tangors the two seasons are remembered together as the Time of the hot dark Death, yet from the fourth to the sixth month after the event there are some changes: temperature will decrease, but not as much as in the north; visibility will decrease too and eventually most of the vegetable life will die, and consequently many animals too.
Encounters and special events of the Black Rain could be used here too, with slight modifications, particularly as survivors become more and more desperate, undead and millenarian cults will start to spread, and the Togoro will find quite difficult to maintain order even in the capital, while communication with other cities and regions of the Empire is sporadic at best. Six months after the event temperatures will drop even in Tangor, and so will begin the..

Time of the cold dark Death
Seven to twelve months after the event, Tangor city area

Without sunlight, beneath and over the now huge mass of dead vegetation and animals, some lowlife will start to prosper, mostly mushrooms, insects, small lizards and mammals. Unfortunately for now this life is composed mainly by little creatures, not very adequate as sources of food. 
Food will be the main issue in this time, as many communities of the kingdoms are starving because were unable to salvage enough vegetation or farm animals, and will start to flock toward the capital city, seeking the help of the Togoro. The situation is worsened by the snowstorms, as temperatures go down to -5°C (23°F), an incredible weather never experienced before by tangor people. Snow and ice will not cover the earth as in the north, yet cold will be a great danger in a land where heavy clothes are not common and vegetable and animal material to sew them is getting scarce.
The undead threat will continue to worsen as dark and despair create the ideal conditions for the spreading of death cults.
The Wasting will begin to appear in Tangor too, creating even more panic and fear.
After six months of snow the temperature will increase a little, even if it will remain way below the average of the tangor region before the event for the following nine years, and the situation will improve greatly, starting the...

Thirty One Night Wars.
From one year to ten years after the event, Tangor Empire

In Skothar the Tangor area, even if devastated by earthquakes and the other effects of the event, is still the only region not entirely covered by snow and ice. The only region where there is life above the ground, where mushroom forests are growing and the temperature is suitable for animal life. That's the reason why refugees from all northern and central Skothar are coming here. 
Tangor chronicles will remember these days as The Thirty One Night Wars, as invaders with nothing to lose from all the rest of the continent tried to conquer the kingdom. Humans of neathar, oltec and mixed descent, rakastas, gyerians, centaurs, gatormen, frogmen, beastmen, undeads, formians, hivebroods, elementals, demons, giants, dragons and many other bands of mixed races and creatures assaulted the kingdom over and over, in fact besieging it from all sides and for nine years. The Empire survived thanks to innumerable feats of valor and diplomatic genius by those who the chronicles remember as the Chosen, in reality some of the random individuals that the Togoro picked up right after the event, assigning them the most different tasks. Some heroes defeated armies, some found precious allies, some gathered food and water. The Second of The Seven Tangor Empires was born here, as the four elite units of the Empire's army that exist to these days, the Dragon army, the Rakasta army, The Allies army and the Noble army. 
(Obviously that's only a possible scenario. If the PCs are unsuccessful the Kingdom could be completely destroyed, decaying in warring tribes and multiple invasions that will forever change the ethnic composition of the region). It's probable anyway that Tangor city will be sacked, conquered, occupied and liberated multiple times during these nine years. 
The invasions will stop (momentary) only with 

The Drowning
The spring after nine years of invasions, Tangor Empire

The heavy rain of this season transform all the Tangor area in mud, making movement impossible for any hostile army. So the Empire has a respite from refugees and enemies, and is able to reorganize itself, even if mostly on boats, as that's a season of flooding.
PCs could have much to do anyway, building ships, reasserting imperial authority in remote areas, fighting scattered bands of marauders and pirates, strengthening diplomatic relations, helping people all around the realm. Compared to the previous years this season is almost a joyous prelude to, finally... 

The Dawn
And the Day did come, Tangor Empire

When the rain end the Empire will have survived one of the most dangerous time of its long history, and will be forever changed by it, in landscape, in internal organization and in ethnic composition. The DM could continue the campaign through the history of the Empire, that will fall five more times till the modern age, under hivebrood, jennite, humanoid and other invasions, yet will always be reborn from its ashes. 
And that's all for Tangor, now we'll go to Oltec and Azcan lands...

The Fall of the Fourth Sun
The sunset and the first night after the event, Azcan and Oltec cities.

It was sunset in northern Brun when the event hit Mystara, so for Azcan and Oltec people the Great Rain of Fire began literally with the Fall of the Sun. The sudden, brighter light, the deafening noise and the subsequent deadly red sky were later interpreted as the time when the Sun god punished men because them, with their wars, were hurting the earth. 
As in other parts of Mystara, everyone not protected by a building, heavy foliage of water was killed by the red radiation, and even some of those who find shelter were killed anyway by meteors, fires, earthquakes, lava or tsunamis. 
The day after no sun rose and so the survivors called this time

The Dead Light.
The ten years after the event, Azcan and Oltec region

Conditions in Brun in the three months after the event were similar to those in Tangor: dim light, heavy black clouds, nonworking and unstable magic, difficult breathing, gasses, hot climate, yet despite the huge fires and the terrible destruction, not all animals and plants have died, at least for a time. Differently from Tangor, however, the social and political structure of the Oltec and Azcan Empire, already fragile due to the war, has collapsed completely, so there is no one organizing and helping survivors, unless PCs do it. Death cults spread in this region more quickly than in other places, due to the crippling despair that dominates among the general population. 
We know from Intua and Atruaghin history that the Azcan will end up hiding underground for centuries after the Great Rain of Fire, this will happen quickly after the first months of eartquakes, acid rains, meteors and poisonous clouds. The prevalent opinion among survivors will be that only underground they could survive the ongoing wrath of the Immortals. 
The Azcan empire at its height was not very friendly with its neighbors, so other races living in the area at this time, like beastmen, giants, dragons, fairy folks, chameleon men and neathar humans will be hostile toward surviving azcans. An isolationist and racist attitude will prevail among azcans, leading them to avoid any other race and hide under the earth.
The story is different for Oltecs. In canon and fan texts there isn't a clear history on the fate of the oltecs left in the Outer World, except maybe that some of them will became the modern populations of the Sind desert, the Savage Coast and the Arm of the Immortals.
PCs could lead one of such groups, among all the difficulties already described for Tangor, with some differences: as this area of the world will become warmer after the cataclysm due to the axis shift, the years long winter is less severe and snowstorms are rare, even if a deep layer of dark clouds will still make photosynthesis impossible, with the result of a mushrooms forest growing. 
So in the ten years of The Dead Light PCs will have to forge alliances with other peoples, like tortles, elves, lupins and rakastas, or even beastmen and araneas, and fight marauders and undeads. An hivebrood invasion is also possible, but the undeads should probably be the biggest danger for the survivors who do not wish to become one of them. 
In this area too the long dark winter will end with heavy rains and extensive flooding, until finally the day will come with...

The Birth of the Fifth Sun.
Eleventh year after the event, former Oltec empire region. 

When the situation will become slowly normal again PCs could help survivors found a new nation in the area or even lead a migration to other places, like Davania or Skothar. In canon and fan text there aren't, as far as I know, stable oltec nations in western Brun or northern Davania after the Great Rain of Fire, but PCs could easily change this, as the remote history of the area is largely underdeveloped. 
And now to Evergrun...

The day of Nature's wrath
The day and the night of the event, Evergrun

It was a peaceful late morning in Evergrun. The great elven capital was one of the biggest city of the world, full of technology and magic, yet it was still a beautiful garden compared to human cities. A garden bustling with activity in that time of the day. When the great noise and the great light came, the city population had the time to feel fear, doubt, horror, blame humans and seek cover before the arrival of the great wave of fire, yet many died anyway, struck by earthquakes, meteors and the horrible red radiation. The survivors wept bitter tears, seeing their beautiful city almost destroyed.
As the Elven empire was powerful in magic and not directly hit by the event, much more people than in other nations survived the disaster. When the red sky dimmed to black clouds the survivors came out from many hiding places and worked frantically to save the wounded, stop the fires and bury the dead. And for the next three months, the elves fought and hoped that they could, against all odds, save their beautiful nation.

The Bitter Summer 
Three months after the event, Evergrun

With magic working erratically, darkening clouds, earthquakes, meteors, gasses, tsunamis and massive fires in the forest, life wasn't easy for the elves in the three months after the Great Rain of Fire. The Queen was killed in the event, as many of the elders and clans leaders. Yet the young princess and all the elven nation did their best to save their people, the forest, the animals, even the neighbors races with whom they had friendly relations, as oltec humans, rakastas, dragons, sphinxes, enduks, halflings and fairy folks. At the end of the season, as bitter and difficult it was, many in the elven kingdom had started to hope that the worst was behind them, and Evergrun would soon raise again. Skyships were even sent abroad to investigate the situation in other areas of the world.
But the elves were wrong. The worst had yet to come and there was no hope for Evergrun.

The Coldest Death
Four months after the event, Evergrun.

The Great Rain of Fire brought Evergrun to the South Pole. Probably that didn't happen in a day and not even in some months, but in many years, yet at the beginning of the fourth month after the event, the first effect of the axis shift became evident in the elven nation.
A incredibly massive snowstorm hit the nation, burying Evergrun under meters of snow in the matter of a few days, killing many, driving animals away, freezing any plant to death.
Barely one week later, the situation was already unbearable. Except for a minority of stubborn technomages and some elite troops of the Empire, the majority of the elven population left the freezing land to reach Grunland. Before the Great Rain of Fire the major elven sister nation was to the south east of Evergrun, but now it was shifting to the north east, and the local elves had told evergrunians that the cold was less severe there.
Battling snowstorms and marauding monsters, the elven populations reached Grunland in some weeks, leaving many along the way. 

The Winter
From the fifth month to 9 years after the event, Grunland.

Life in the now mushroom forests of Grunland, under the dark sky, wasn't easy in the nine years after the event. Discord among the elves was rampant, mostly among the first returnists and the government, but also within the government among the Evergrun refugees and the elders of Grunland. The only thing that kept the elves united in spite of the many conflicts were the numerous external threats. The cold, the snowstorms, the earthquakes and the massive extinctions of plants and animals were the worst, but Grunland suffered also many attacks from desperate humans, giants, dragons, former serpentine troops and many other creatures, attacks that prompted yet other conflicts between the elves that wanted to kill any outsider and those who wanted to seek alliances with other races. Except for halflings and gnomes, the mounting paranoia in Grunland often lead the elves to attack other races on sight in this hard times.
Eventually the elves of Grunland managed to survive, but the discord of these years planted deep seeds that will result in Ilsundal and Atziann migrations. 

The Dawn
Ten years after the event, Grunland

The elven nation that emerges from the disaster is a divided one, with many unresolved questions that will plague it to its eventual dissolution. Even when the environment situation becomes more and more normal, earthquakes, volcanoes, external invasions and internal discord will continue to haunt Grunland to its inevitable demise...
To be continued, underwater, among the Aquarendi...

The Fiery Dawn
Aquarendi underwater kingdom

It was dawn at the time of the event in the Aquarendi kingdom and PC3 The Sea People has a pretty good description of the first day of the Great Rain of Fire as experienced by the aquatic elves: “The sky turned yellow, red and then black. Ash obscured the sun... dark waves lashed against the land, and the canyons of the Deep split open and gouted out fire and destruction”.
Aquatic creatures were mostly protected from the deadly red radiation, yet volcanic eruptions and tsunamis brought much destruction and death even undersea. Then, as PC3 says, the ocean stilled, and the elves thought the worst was behind them. But an horrible realization hit them as soon as they went in search of their land brothers of Evergrun. The once beautiful forest was a burned, destroyed wasteland, with not one animal and one plant left alive. The Aquarendi searched for weeks but, unable to go too much miles beyond the coast, they could not find any proof of surviving life. Then the snowstorms began and they were forced to drop any search and immediately return to the sea. They thought themselves safe under the waters, but soon the oceans too were hit by the great cold.

The Great Cold
Under the seas of Davania's western coast, the nine years after the event.

The seas had grown cold, and the land was covered in ice. The peoples divided and sought new homes along the warmer currents that flowed from the north”. 
The aquatic elves and the other underwater races with whom they had lived alongside for a hundred years began separate migrations to escape the cold, finding temporary homes that were soon destroyed by volcanoes and tidal forces. Only centuries later they will be able to reach the Sunlit sea, discovering the Meditor and Verdier elves already living there.
In the first ten years after the cataclysm, with mass extinctions in the seas, the sky covered by dark clouds and any coastal vegetation dead, the aquatic elves have every reason to believe that life on land was totally extinct, and survived just underwater.
Only with the Dawn they found proof of life above the waters, when plants began to grow again. Before that, the acquatic elves life was a long, hard migration in constant peril, searching for food and shelter, battling elements and monsters. 

The New Dawn
Under the sea of the North western coast of Davania, ten years after the event.

When the Sun returned the aquarendi found a home in this area of the world, along with merrows and other aquatic people, at least for a time. They also encountered, for the first time in ten years, land survivors of the Great Rain of Fire, enduks, dragons, sphinxes and elves of the former Golden Empire, with whom they established peaceful relations.
This time of peace will eventually end when seismic activity will force the aquarendi in a new migration, toward the Sunlit sea...

And that's the end, for now, of these ideas about adventures and campaigns under the Great Rain of Fire... If I found the time to do it, I'd like to draft a comprehensive map of the world in the final times of Blackmoor, that would also be very useful to understand what modern populations could live today in several areas of Mystara...