Gehenna

From Manuel of the Planes (3e):

It is a plane without charity, mercy, or pity.

It is the Oven of Perdition, the Fourfold Furnaces.

It is where yugoloths cavort on endless volcanic slopes.

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 of Gehenna, 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. Gehenna's 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 yugoloths or deities have a tendency to break apart, sending their builders on a long, sliding fall down the mountain.

Gehenna's four layers are Khalas, Chamada, Mungoth, and Krangath. Each layer is differentiated from the other by its degree of volcanic activity.

Powerful entities that possess realms on Gehenna include many lords of the yugoloths, as well as Melif the Lich-Lord and Memnor, deity of evil cloud giants. The realm of Maanzicorian, an illithid deity, was once located here. But Maanzicorian was slain by Tenebrous, the name taken by the demon lord Orcus when he first returned from supposed annihilation. Accordingly, Maanzicorian's realm has started to crumble, its deity gone.

Each layer of Gehenna (called a mount) is slightly different, but each burns with an evil will. The lava flows seem to seek out the casual traveler, and fissures open under a visitor's feet as if the ground itself hungers. As on Carceri, the sloping earth itself provides light, so shadows stretch upward.