Planes
by Andrew TheisenThe way I do Mystaran cosmology IMC is a modification of the Doppel Cosmology from the Manual of the Planes:
Material Planes
There are two Prime Material Planes, the "Normal" Prime (where Mystara is), and the "Nightmare" Prime (the Gold Box Nightmare dimensions/WotI's Nightmare Dimension). Both have their own Ethereal planes (unconnected to one another), and their own Elemental Planes. Though neither plane has the "Law/Chaos Influenced" traits, nevertheless the Normal Prime has a tendency towards Law, and the Nightmare Prime has a tendency towards Chaos.
Elemental Planes
The Normal Plane has Earth, Fire, Water, and Air Elemental Planes. The Nightmare Plane has Terro, Blackflame, Vischor, and "Nightmair" (pending better name) Elemental Planes.
Ethereal Planes
Each Prime and its corresponding Elemental Planes are encased in their own Ethereal Planes, which are unconnected.
Spirit Plane
The Spirit Plane is a transitive plane that connects the Normal and Nightmare Planes. This is the same "Spirit Yurt" that the Ethengar people have connections with, and it is Coterminous with the Primes. Several areas- many Stonehenge-like formations across Mystara, as well as the mountain in the Land of Black Sand in Ethengar- have direct portals to the Spirit Plane. The Spirit Plane also has connections with other worlds that may not necessarily lie within the Mystaran cosmology (such as, perhaps, Laterre, the land of the d'Ambrevilles).
Astral Plane
This is the standard Astral Plane of D&D cosmology, though I've altered certain of its aspects to more closely conform to the Astral as depicted in the Gold Box Immortals set (such as mortals appearing as two-dimensional beings).
Outer Planes
Number unknown, believed to be infinite. Homes of the Immortals and other godlike beings. I don't use the standard "Great Wheel" cosmology for the Outer Planes, though I may co-opt some of the descriptions of the planes (and names) for use as homes of the Immortals.
I don't use the Negative/Positive Elemental Planes- negative energy is a natural occurrence in the universe (from which the Sphere of Entropy gets its name and members of that sphere get their power), and positive energy is a direct by-product of the four non-Entropic elements.
I don't use the Shadow Plane. I consider many/most Shadow planar references (and powers) to derive from the Nightmare Plane (based on a theoretical physics concept I read about, wherein shadows are the result of light reflecting back from the 5th and/or higher dimensions). The Spirit Plane also serves the function of the Shadow Plane's transitive "hop between worlds" shtick.
I haven't quite decided where to place Limbo- the home of dead souls. I kind of prefer to have it be the Spirit Plane (contrary to its description as an Outer Plane), but I'm not entirely sure yet.