I would use Planescape in a campaign where the planes were the primary setting and Mystara was just one of many worlds the characters might visit, but explore Frank Mentzer's distinctive cosmology in a primarily Mystaran campaign. Probably, I mean. Unless I couldn't resist adding in bits about the Blood War and the factions.
It could, yes. I probably wouldn't bother in a campaign where Mystara and those settings strictly tied to it are the only worlds used, unless you really like Sigil. I mean, I like the Immortal's Set cosmology a lot.
I would use Planescape in a campaign where the planes were the primary setting and Mystara was just one of many worlds the characters might visit, but explore Frank Mentzer's distinctive cosmology in a primarily Mystaran campaign. Probably, I mean. Unless I couldn't resist adding in bits about the Blood War and the factions.
I'm not sure. I like Sigil, or at least the concept of such a planar city. Pandius is a bit like that, though given that all the major Immortals hang out there, it is a bit too central IMHO. I like the concept of interplanar cities though, or even just planar ones for that matter. Like the City of Brass, which exists both in the AD&D and Mystara cosmologies.
Håvard
- Sigil remains a 'gateway' city, providing easy access to many of the Outer Planes. To keep its wonder and mystery in a multiverse in which most Outer Planar locations are created or at least found by the Immortals, Sigil becomes one of the few places in the multiverse where portals and vortices can actually spontaneously form. It is not necessarily 'central' to the Outer Planes (as finding a center to infinitely varied space is difficult), but it comes closer than anything else in the multiverse.
- The Lady of Pain remains a power within Sigil, though she is less concerned with matters of balance or control over the known denizens and presumably more active in defending the city against possible beings from Beyond (the Outer Beings, certain forces of the War of the Years Before Eternity, and so on). Her origin is unknown, but she is definitely not an Immortal; some Immortals speculate that she is a creature or agent of the Old Ones, or a manifestation of an Old One herself.
- Immortals are not automatically barred from Sigil, though most who choose to visit do so in Mortal guise in order to keep a low profile. The Lady does discourage Immortals from beating on mortals, in her own uniquely charming fashion. Immortals generally have their own ways of travelling the planes, but sometimes choose to use Sigil as a meeting ground or shortcut.
- Mortals in Sigil are wanderers, exiles, sometimes dispossessed from their home worlds or times - exceptional in every way. They do not come here for opportunity or wonder, generally, but because they have nowhere else to go. Factions and sects are less about manipulating belief as a force that defines reality, and more a tool for insuring that certain individuals are on your side in a dangerous and uncertain reality. The kriegstanz is as strong as ever, with not just prestige at stake but often life itself.
- Among the places that Sigil opens onto are the plane of Limbo, select locations in the Nightmare Dimension and the rare portal to the depths of the Vortex. This leads to bizarre 'rescue' missions staged by mortal, Nightmare being and Immortal alike, as they seek to use a unique opportunity to reach an otherwise-inaccessible location to rescue a friend or doom an enemy.
How useful is Planescape (or even 3E planar) material for Mystara campaigns? IMO, combining Mystara with the other standard TSR settings (FR, GH, BR etc) is difficult to accomplish without diminishing Mystara's unique features. OTOH, Mystara still shares many things with the other settings. Can Planescape be reinterpreted for exclusive use with Mystara?
Most Planescape stuff deals with campaigns centered in the Outer Planes, with a point of view different than those of the other campaigns.
So, combining it with Mystara is as difficult as it would be for any other setting.
Mechanically, there are several differences:
1) Mystarans pass through "Limbo" when they die, not through the Astral as their AD&D counterparts. This Limbo is not the same as the Outer Plane of the same name.
2) The elemental oppositions are different in OD&D.
3) The structure of the Outer Planes is different -- PS "Realms" are Mystaran "Planes".
Most of them are easily overcome:
1) Mystaran Limbo is a part of the Ethereal between the Prime and the Negative Planes. The fact that Mystaran deaders go through this plane rather than through the Astral has parallels in Dark Sun (and, to a certain extent, in Birthright).
2) The elemental oppositions as seen in Mystara are a skewed perception due to prevalent philosophical trends in Mystara and/or to the ideals of the elemental rulers that interact most often with Mystarans (here as well the parallel with Dark Sun, with its variant paraelementals is useful).
3) Mystaran "Outer Planes" can easily be considered as Realms -- that individual powers have their own Outer Plane may simply be one of those weird clueless ideas. The main planes of each Sphere would simply be regions of the Elemental Planes.
BTW, note that Mystara is more connected to the Elemental Planes than to the Outer Planes (there're many well known passages to the Elemental Planes, and almost no passage to the Outer Planes), and many Mystaran Immortals have a strong elemental nature.
This makes Inner Planar stuff more useful -- I'd allow genasi PCs in certain areas of Mystara, for example, and Inner Planar magics would be thaught at the Great School in Glantri.
In the end, the real issue is whether you want Planescape themes to bleed into your Mystaran campaign, and/or your Mystaran PCs to become involved in the Sigilian/Outer Planar events (factions, Blood War, etc).
If so, then you could use Planescape easily, and you'll find that several factions will have an interest in Mystara -- more than they would have with other prime worlds -- due to the Immortals (e.g., the Athar and the Believers, but also the Doomguard).
Otherwise, it would be best to use only the Inner Planar resources, and leave the Outer Planes in the background -- or simply use the OD&D Outer Planes.
Basically, it is an inn with more than 100 doors leading to every part of the universe(s) and with some very unusual guests :D
:-) Jesper
One big difference between Planescape and Mystara is that in Planescape it's very important that entropy isn't an inherently evil concept - the Doomguard are an inner planar faction who protect the natural process of universal decay. Some of them interpret this in an evil concept, but not all. In Mystaran terms, there are as many Doomguard dedicated to the Sphere of Time (or even Matter) as Entropy.
Mystara's Limbo somewhat resembles the Fugue Plain that the souls of Forgotten Realms characters travel to after death. In 2nd edition AD&D, the Fugue Plain was a realm in Hades, and Limbo could fit in the same way.
As far as the main planes of each sphere go:
Arborea: Energy, Thought
Beastlands: Thought
Elysium: Thought, Time
Bytopia: Matter, Thought
Celestia: Matter, Thought, Energy
Arcadia: Matter
Mechanus: Matter, Time
Acheron: Matter, Entropy
Baator: Entropy
Gehenna: Entropy
Hades: Entropy
Carceri: Entropy
Abyss: Entropy
Pandemonium: Entropy, Thought
Limbo: Energy
Ysgard: Energy, Matter, Time
Arborea: Energy, Thought
Outlands: Time (Pandius rests at the base of the Spire, and this region belongs to no Sphere).
Individual realms might be ruled by any sphere, or no sphere.
And then the Inner Planes:
Positive Energy Plane: Energy, Matter, Time, Thought.
Radiance: Energy
Lightning: Energy, Thought
Steam: Energy, Time
Mineral: Energy, Matter
Fire: Energy
Magma: Energy, Matter
Earth: Matter
Ooze: Matter, Time
Water: Time
Ice: Time, Thought
Air: Thought
Smoke: Thought, Energy
Ash: Energy, Entropy
Dust: Matter, Entropy
Salt: Time, Entropy
Vacuum: Entropy, Thought
Negative Energy: Entropy, Time, Thought, Matter, Energy
Demiplanes:
Electromagnetism: Energy
Time: Time
Ravenloft: Entropy, Thought
Other Planes:
Region of Dreams (Curtain of Colors): Thought
Temporal Prime: Time
As far as the main planes of each sphere go:
Thank you. Nice to have.
My mystara campaign uses Planescape cosmology. Simply cause of the time I begun DM'ing Mystara the only material I had about it was "Kingdom of Adventure"-box. And I was really into planscape at that time, so I early on introduced Planescape-elements to the players. Later on getting more material on Mystara I simply got headaces from my priviously mistakes. Now I've made some cubtle changes for my campaign to better fir what is described. But mostly I just let it be and don't think to much about the big questions. The players don't ask anyhow and is satisfied with the Planescape stuff it seems.