After waking up this morning, I started thinking about the planes of hell. Such is the life of a Dungeon Master ;)
In contrast to the traditional AD&D conception of the planes of existence, in my campaign I've decided that there's only one plane or world, and that all deities, devils, demons, etc have their realms located physically in that one world. (I don't claim to know a huge amount about real-world mythology, but I think this was pretty much the world-view held by people such as the Norse and the ancient Greeks.) Of course, most of these divine or infernal regions of the world would be obscure, mythical and pretty much impossible for average mortals to reach. (Heroes with powerful sorcery and magic items, on the other hand...)
So, in my campaign, the spirits of the dead travel through the Underworld (literally underground) and have to run the gauntlet of spiritual worth as various unsavoury beings vie to steal their souls. Up until now I've not put that much thought into the dark forces of the world, other than creating a list of the names of some of the major gods of Chaos - who'd be exactly the folks who would be ruling these subterranean domains.
This is what came to my mind, inspired by the idea of "the nine hells" (not sure if that's an AD&Dism, or if it's based in real-world legend?):
The Nine Hells
1. The burning hell
2. The freezing hell
3. The boiling hell
4. The parched (or desolate) hell
5. The caustic hell
6. The lightless hell
7. The hell of hooks
8. The hell of holes
9. The hell of dominion
Each of them of course now needs a suitably nasty ruler and his or her servitors. For example, I imagine the boiling hell being a swamp-world filled with bubbling pools and noxious vapours, ruled by a frog god and his amphibious devil servants. The ninth hell I envisage as the kind of central hub (also perhaps the lowest), from where the as-yet unnamed lord of hell rules each of the other eight.
Fun times :)