Are they spending time in the Astral plane, or the Outer planes?
Because the Astral is a transitive plane. It's mostly empty silvery space with a few pockets of "stuff" that spills over from the planes it touches. Think along the lines of what space is like for us in the real world.
Not that it's bad for adventuring, just that it's not really full of stuff like Outer Plane traveling... so more astral creatures and other denizens (githyanki/githzerai, etc), and less about demons or angels.
Some info you should probably read up on before starting is in the Game Mastery Guide, I believe. The info is open source on their PRD, here:
Planar Adventures.
Specific info on the Astral Plane:
Astral Plane
The Astral Plane is the space between the Inner and Outer Planes, and coterminous with all of the planes. When a character moves through a portal or projects her spirit to a different plane of existence, she travels through the Astral Plane. Even spells that allow instantaneous movement across a plane briefly touch the Astral Plane. The Astral Plane is a great, endless expanse of clear silvery sky, both above and below. Occasional bits of solid matter can be found here, but most of the Astral Plane is an endless, open domain.
The Astral Plane has the following traits:
- Subjective Directional Gravity
- Timeless: Age, hunger, thirst, afflictions (such as diseases, curses, and poisons), and natural healing don't function in the Astral Plane, though they resume functioning when the traveler leaves the Astral Plane.
- Mildly Neutral-Aligned
- Enhanced Magic: All spells and spell-like abilities used within the Astral Plane may be employed as if they were improved by the Quicken Spell or Quicken Spell-Like Ability feats. Already quickened spells and spell-like abilities are unaffected, as are spells from magic items. Spells so quickened are still prepared and cast at their unmodified level. As with the Quicken Spell feat, only one quickened spell or spell-like ability can be cast per round.
That "auto-quicken" magic thing will give the casters a leg up over Martial classes. You'll have to keep an eye on Martial characters, because they might feel a little more "second fiddle" than before compared to any casters in the group. Perhaps some artifact-level equipment (weapons, armor or macguffin) would be appropriate... they usually have a plot-sword or somesuch by normal 20th level gaming anyways.
I'm assuming they are starting at 20th level, and not characters they've played from 1st to 20th already..
The subjective gravity though, could help:
Subjective Directional Gravity: The strength of gravity on a plane with this trait is the same as on the Material Plane, but each individual chooses the direction of gravity's pull. Such a plane has no gravity for unattended objects and nonsentient creatures. This sort of environment can be very disorienting to the newcomer, but it is common on “weightless” planes.
Characters on a plane with subjective directional gravity can move normally along a solid surface by imagining “down” near their feet. If suspended in midair, a character “flies” by merely choosing a “down” direction and “falling” that way. Under such a procedure, an individual “falls” 150 feet in the first round and 300 feet in each succeeding round. Movement is straight-line only. In order to stop, one has to slow one's movement by changing the designated “down” direction (again, moving 150 feet in the new direction in the first round and 300 feet per round thereafter).
It takes a DC 16 Wisdom check to set a new direction of gravity as a free action; this check can be made once per round. Any character who fails this Wisdom check in successive rounds receives a +6 bonus on subsequent checks until he or she succeeds.
This can mean constant full attacks, since movement is based on "falling" and not actual movement spent. If they are doing combat within a small enough area, they can pretty much do "run by attacks" without actually using a move action.
DC 16 Wisdom check may or may not be a toughie for some martial characters, make sure they are aware of this before they head into the environment. Rings of featherfall (or some other permanent form of it) would probably also be a good idea, just in case.
Other thoughts:
1. Keeping track of a 3-d environment on a tactical grid might be tough. Since most of the environment is empty except for the pockets of material, you can probably play without a map for much of combat.
When you do need to keep track of positions in a more exact way, you could resort to clear sheets to indicate who's "above", "in the middle" and then the normal grid for those on the bottom. This works better with paper squares for minis.
Alternatively, if you don't have see through material, you can draw three version of the map, moving people to different ones as they move up and down.
2. Emphasizing the "alien" mindset of the creatures that exist in the Astral plane is a good idea. Without need for food, and being ageless, diseaseless, etc, any interactions with local denizens should be sort of odd. That and the gravity thing... navigating places more akin to an M.C. Escher painting than anything we experience normally should be pretty mind blowing.
Also, since this is the transitive plane for souls going to their afterlife... with creatures that feed on them specifically if I recall correctly (those astral beasts)... "souls" could be a much more common currency for 20th level transactions than any actual gold or material worth. Which brings a whole set of moral dilemmas for those not used to this kind of thing. Allowing research for an epic spell to see and capture a soul could be an idea. Or allow the Soul Bind spell to react differently when in the Astral Plane?
3. Spells would react mostly the same, although those dependent on certain environments might not be as effective.
The Reverse Gravity spell would probably work a little different from normal, in that it would just create an area (sized per the spell) that reverses whatever gravity a person has when they enter it, effectively trapping them if they aren't moving fast enough to move through the whole thing in one round.
Hope this stuff helps!
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