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Mystara cosmology and 3E - astral planeby David Knott
From the point of view of those on the Mystaran Material Plane, the main distinctive factor of the Mystaran Astral Plane would seem to be its lack of any direct connection to the Mystaran Material Plane.
As a result, Mystaran adventurers seldom if ever astrally project themselves onto that plane -- more typically, they physically enter the Ethereal Plane, cast themselves off into the Deep Ethereal, and then ultimately find a portal that enables them to reach the Astral Plane physically. From there they are able to reach the Outer Planes associated with the five Spheres (Brynn, Draesten, Entrem, Mirage, and Pyts) or alternate cosmologies (Nightmare, Myth, and perhaps Vortex).
The Mystaran Astral Plane has the following traits:
Dimensional shifting: Because the Astral Plane is shifted one dimension away from the orientation of either the Mystaran or Nightmare cosmology, mortals from either of those cosmologies appear two-dimensional on the Astral Plane. Spells cast by them have their effects show up perpendicularly to them (but still two-dimensional). As a result, mortal spells are particularly easy to avoid -- treat any target of a spell cast by any mortal being on the Astral Plane as having the Improved Evasion rogue class ability even if he does not. Combatants from the Mystaran and Nightmare cosmologies remain completely immune to each other's spells on the Astral Plane.
This ability does not apply to Immortals and their Exalted level minions, as such beings and their spell effects are fully three dimensional on the Astral Plane. WotI suggests penalties to saves vs. such spells, but I suspect that letting their spells function normally would be penalty enough for dealing with their spells on the Astral Plane.
Connections to other planes: The Astral Plane is the preferred way to access other planes and cosmologies. While most outer planes are accessed by passing through the main outer plane of a given Sphere and then moving on to the plane of interest, there do remain other planes (such as those of the Draconic Cluster) that are accessible directly through the Astral Plane. The dimensional shifting of the Astral Plane clearly suggests it as the obvious way to reach the Nightmare cosmology. As for the Vortex cosmology -- the route to that cosmology is not known even to the Immortals, but the description of the Far Realm plane suggests that it can be reached from the Astral Plane and that anyone who can reach that plane and survive may be able to travel beyond it to the Vortex cosmology.