Gehenna

Gehenna is a cavern. One impossibly large, and with the floor and ceiling possessing their own gravity (and thus being habitable). The only connection between the two sides is through a number of pillars, in which many kilometers long stairs have been carved. There might be an end to the cavern, however it is hard to say for sure - when you go far enough and step beyond the threshold, you can expect the Reality itself to go wonky, and going further will have you reenter the 'stable' part from a completely different direction. Threshold appears to be moving outward, as the population of this realm grows.   There is almost no light in Gehenna - Lucifer's death took all of it with him. There is some leftover ambient light, just enough for the entire realm to not be pitch-black, but it's still mostly on the level of a moonless night on Earth. Rest is either natural light sources (rare, since the local inhabitants can see in darkness and tend to dislike light), or the glow of lava lakes and rivers and other fiery pits of geothermal energy that are so common in Gehenna (as a side effect, it has some truly wonderful hot springs, just don't expect them to be tourist destinations).   The most notable point in Gehenna is the Pandaemonium, once the seat of Lucifer himself. It's less a city and more a geographic feature - a massive, vertical city build in place of some particularly wide pillar connecting both sides of Gehenna, with the Lucifer's throne room placed perfectly in the middle of it, on the intersection of two gravity vectors. For reasons unknown, the throne room remains sealed ever since its owner's death, but Pandaemonium itself remains a city and an unofficial capital of Gehenna - a neutral city where the demon lords come to parlay and trade without a threat of backstabbing or enemy assault..