Frostburn campaigns in Mystara
by John Calvin1) The Death of the Sun
For some reason the red sun in the Hollow World goes out. Perhaps the Zargosians finally succeed in their plans, or perhaps Thanatos manages to build (or rebuild) his Smoking Mirror (or some variation thereof), but whatever the cause is, the HW is thrown into darkness and the temperatures drop rapidly.
Since the Spell of Preservation was powered in great part by the HW's sun, this also has the immediate effect of removing many cultural biases from the peoples of the HW. This is a good thing since many of those cultures are tropical, or desert dwellers, who must adapt to the cold as quickly as possible or freeze to death.
The first portion of the campaign would be playing through the apocalyptic events of the freezing. The heroes should quickly rise as leaders of their respective nations and try to find ways to preserve their culture and their people. Many groups might try to take advantage of the newly dark HW including the Zargosians, the Schattenalfen, and perhaps the Bahlor Empire. Some of the burrowers might be able to free themselves from their long slumber. Perhaps without the SoP in place some ancient terror is able to free itself from the Blacklore valley, or from Blackmoorian Jomphur. All of these threats would need to be dealt with in order for the HW cultures to survive intact.
The second portion of the campaign could focus on the cause of the Red Sun dying, and perhaps PCs could work toward its eventual resurrection. If this is not possible, then their might be one more course of action for the PCs to take. Once the red sun dies, the only source of light in the HW will be a faint glow that radiates from the cloud covered polar regions. This is caused by sunlight from the outer world diffracting in through all of the fog and clouds of the poles. In the OW summertime, this light comes in through the north pole and in the OW winter it enters through the south pole. If all else fails, then PCs might be tempted to lead their people to the only source of light left...and eventually find an escape from the HW!
2) Invasions From the Past
This campaign option assumes that the events from campaign #1 above have already taken place. As the winters grow longer, and the polar ice caps creep further and further toward the equator, the nations of the Outer World find themselves being invaded by ghosts from the past... or so they think. In reality the HW nations have found an escape from their freezing tomb and are intent on reclaiming the Outer World for themselves.
The PCs will be tasked with meeting the threat that advances from the poles. The Azca, the Antalians, the Krugel and beastmen hordes, Jennites and Tanagoro warriors, the Milenian Empire... and even Lost Nithia! Depending on the timeline, the Alphatians themselves might make an appearance!
The first part of the campaign will consist of the adventurers discovering this new threat and trying to stop it. However, their adversaries are desperate to say the least. There is no turning back for the peoples of the HW, and they will fight to the bitter end if need be. If played just after the end of the Wrath War, this can be devastating for the Known World nations as they must recover from their wounds or be shattered by this new threat.
The second part of the campaign will begin when the PCs realise the reason for the invasions... that the polar ice caps will quickly expand to cover their entire world. Can they stop the real threat before it is too late? Perhaps they will be able to recruit allies from the HW nations.
3) War of the Worlds
Building from the previous two campaigns, we continue with an invasion from another world. Deep in the heavens, a very long way from Mystara, another planet in the Solar System has just lost its long battle. In the beginning M-Mars was very similar to its sister world Mystara. It was a vibrant world, with advanced civilisations. They built colossal canals across their continents, and gigantic cities dotted the landscape along them. Then the Others came...
The world became darker and colder, with each passing year. At first it was imperceptible to the natives (who I picture as Egyptian type Aranea), but soon the growing cold was too hard to ignore. By then it was too late. Creatures from the elemental planes poured out across the landscape, and began to conquer and enslave the populace. Now that they have won the war on M-Mars, they turn their attention to Mystara.
Perhaps the first wave of invasion could come from the M-Mars natives as they desperately try to escape their doom. Perhaps they are the ones who lead the Others to Mystara in the first place? Regardless, the PCs must now fight to save their entire world, lest it suffer the icy fate of M-Mars.