|The whole solar system.|
Gaianna itself is only partly habitable with advanced life an mankind dwelling in great seed-craters full of water and lush life, outside the crater walls a desolate landscape where the air hangs low and requires extraordinary means to survive.
The scholars of the Gaiannan civilization are ignorant of their true place in the universe. They believe their own crater to be a vast fertile valley ringed by the mountains of the world beyond which lies the disk of the outer world a vast desolate realm hostile and unsurivable to man; hell is just a long walk away.
Magic and other forbidden sciences allows contact with a host of beings that live elsewhere in the system and perhaps beyond.
|A very rough map of the surface, I'd start the players off in that middle crater.|
So there ya go. PCs are residents of a postapocalyptic galactic civilization currently isolated on a massive terra-formed domain they believe to be an entire world in an of itself. Demons and such are natives of other worlds on this system and beyond. Adventure sites would be in each of the seed-craters on Gaianna and a few scattered about the system.