Gehenna (Bleak Eternity of Gehenna, The Fourfold Furnaces, The Fires of Perdition)
Gehenna’s top layer borders Hades and the Nine Hells, so it is not a pleasant place. Floating in an impenetrable, infinite void are volcanic mountains seemingly without base or peak. They are only finite in the strictest sense of the word, measuring hundreds of thousands of miles in each direction. A single volcanic mountain dominates each of the four layers, though lesser volcanic earthbergs drift and sometimes smash into the greater mountains.
There is no naturally occurring level place in any of the layers; all the slopes are at least 45 degrees, and many are akin to sheer cliffs. Its fiendish inhabitants have carved artificial ledges, some large enough for entire cities, and switchback paths to connect them. But those edifices not carved by native Daemons (Yugoloths) or deities have a tendency to break apart, sending their builders on a long, sliding fall down the mountain.
The plane consists of four planar layers; each layer consists of a pair of huge volcanos floating in space, joined at the bottom to form massive “earthbergs”, hundreds of thousands of miles tall. Smaller mountain-shaped and -sized chunks of earth also float through Gehenna, occasionally colliding with the primary earthbergs. The first layer, Khalas, is a hot and fiery place, but the volcanos gradually grow colder as one passes through the layers; the third layer, Mungoth, is a cool place, while the fourth, Krangath, is icy and dead.
The Crawling City
This city isn’t specific to Khalas, as it actually traverses all of Gehenna since it’s mobile. It is the home of an extremely powerful Ultraloth, known only as The General of Gehenna. The city itself has massive demonic legs grafted onto it which are able to resist the heat of the lava rivers that are all over Gehenna. The legs also allow it to climb the steep mountains with relative ease.
The city itself houses mercenaries from both sides of The Blood War. In fact, it plays both sides of the war fairly well. Housing elite crafters who will happily sell their advanced hardware to whatever side is paying the highest. The city also has a prestigious War Academy where officers from both sides of the war learn tactics and lore to help them in their conquest. Legends say that if The Crawling City ever chose a side, then the Blood War would reach its final apocalyptic end.