Despite the canon TSR material, some Spelljammer players and DMs want nothing more than to visit the uninviting world of Athas, home of the Dark Sun Campaign. Presented here are ideas on the nature of the sphere in which Athas, the world of the Dark Sun, resides.
They did it, I cannot believe, they really did it!
Athas IMC was not a sphere called Athaspace... but REALMSPACE! Athas always fit the Sci-Fi genre of post-apocalyptic earth. With a city called Tyr... Tyr being one of the major gods on Toril, I decided it was Toril that got blasted. The conversion wasn't perfect, but there was some things for me to play with. The elves on the Poscadar Plains in Maztica are very similar to the elves in Athas, and of course there were Thri-kreen on the Poscadars as well.
Any way, what happened was one of our arch-foes got hold of a witchlight marauder and bombed waterdeep with it. Conveniently, Elminster was on one of his excursions away from the planet... The Marauder was far too powerful and unstoppable before anyone could gather up a defense. When finally the Witchlight was defeated, the planet of Toril was a blasted ruin.... The survivors abandoned their gods in anger, turning instead to a cult-like worship of elements. With Mystra gone, the nature of magic changed and those that practiced it anyway simply made things that much worse for their world.
Our characters had run afoul with some villains in the phlogiston, and ended up floating for a thousand years. When they awoke, they were on the surface of this strange world. It turns out that the Druid of the Forest in one of the DS adventures sensed their arrival through a natural portal and teleported them to the surface. They were captured and escaped from Tyr, found the Druid (who was Elminster, charged with protecting and healing the sphere by Mystra before she and the other gods left) and went back in time with the help of the space-time portal in the Black Spine mountain adventure. Our party was charged with stopping the villain that started the whole cycle by preventing him from dropping the Marauder.
This is just a brief outline covering only the major things that had to change... I had something push the planet alittle closer to the sun...etc.
Since I am not a particular fan of the "closed world" stuff sometimes done with ADnD, I have defined the basics of the sphere, but is a very tough and harsh sphere and going there will most likely crash you on one prime world, where you do not want to be.
Surprisingly the sphere is dominated by a vast crimson sun. (It is a heliocentric sphere) It actually encompasses almost a fifth of the available space and erased several of the inner worlds, when it expanded. There are four planets left, but except for Athas they are all barren lifeless husks lacking any type of atmosphere, though one could most likely make a fortune mining minerals there, if it were not so dangerous as to be almost impossible. (Dark Sun material describes Athas in depth, so I will skip it here.)
Only a particularly desperate (to hide that is) spelljammer captain would ever go to Crimsonspace voluntarily, since it is exceedingly tricky and dangerous to navigate. The first problem one is going to face are vast eruptions of elemental fire from the sun, which scorch any spelljammer in a moment, though they are save from them in the outer fourth or so of the sphere. Unfortunately these eruption cause shock echoes, which can hurl a 'jammer even far away from them through half the sphere and easily crush them into the crystal hull or one of the worlds, most likely Athas.
In addition gravity is very unstable within the sphere. Unlike most other spheres a Spelljammer is almost constantly subject to some type of gravitational pull, which can extend from two to twelve times of the normal radius from any of the worlds and changes without warning or apparent pattern. Those few who have survived Crimsonspace with an intact Spelljammer say, that these gravitational pulls can accelerate a craft almost to spelljamming speed and tend to end the unfortunate ship in the orbit of the principal world. In addition craft with weak hull have a tendency to break up in the process, unable to stand the stress of being subject to several competing gravities.
Priest magic will only work up to second level within the confines of these spheres, unless the priest worships an element or elemental power, in which case they cast spells as though they were a level higher.
Wizard magic basically works until one enters the orbit of the principal world, in which case mages have to resort to defiling or preserving (depending on alignment; neutral characters tend to preserve) to memorise / power spells. They may still use these methods of drawing energy elsewhere if they learn its controlled use. Unfortunately there is very little vegetation on 'jammers.
Psionics work normally, or may even be (at the DM's option) be enhanced in some way.
In addition to its effects on magic Athas radiates uncontrolled stabs of mystical energy into space, which are believed to be psionic in origin, but this cannot be proven. Any of these waves will severe the connection between the helmsman and the craft, or forcibly eject the victim from lifejammers and similar devices.
Of course the gravitational pull of the world tends to be in constant flux anywhere from half the atmosphere to three times the world's radius. The power will twist by 0.5 x 1d6 G (G is earth / Toril / etc. normal gravity.) Generally it is directed towards the planets, but this is not necessarily the case. Some people refer to this phenomenon as gravity anomaly.
Surface conditions: Read the Dark Sun Material :)