The Bleak Eternity of Gehenna, The Oven of Perdition, the Fourfold Furnaces or just Gehenna, is not a nice place. Formed out of four impossibly huge volcanoes floating in an endless black void, with little open ground anywhere. The slopes are always in between a 45 degree angle to a full-on cliff face, making falling an unpleasant prospect if one does not want it to keep happening. Aside from the rocky and soot-stained walls, the dangerous terrain (lava, the River Styx, acidic snow) and the hostile inhabitants (Yugoloths and other fiends) make Gehenna a thoroughly unpleasant place best to be avoided.
Layers of Gehenna
Gehenna consists of four layers. Each of them are volcanoes of immense proportions, measuring hundreds of thousands of miles wide. You cannot travel in between the layers on foot: you will need to take one of the many portals to travel between layers and planes. Unfortunately these portals are often at the bottom of bottomless chasms dotting the landscape which you need to jump into and hope for the best. These holes are sometimes marked by the Yugoloth as such, but sometimes they mark actual bottomless holes as being portals. The lava that flows on the layers seems to always flow in the direction of travelers and sometimes the ground opens right before a visitor's feet as if the ground itself hungers.
One of the main sights of the plane is The Crawling City, a giant city with thousands of fiendish fireproof legs on its underside, walking it across and in between the layers of Gehenna. It houses the General, an Ultroloth who theoretically rules over the Yugoloth, but since the nature of the race is that of schemers and plotters they often have divided loyalties. The city itself houses many barracks, siege towers, smithies and other such buildings of war, including the foremost war academy where the Fiends send their most promising generals to train for the Blood War. It has been prophecised that when The Crawling City enters the Blood War, which it has never done before, it will be the decisive battle of the war.
The first layer flows with rivers and waterfalls of boiling hot water, heated by the volcanic activity of Khalas. Part of the River Styx flows through the layer, forming spectacular, huge and dangerous waterfalls. The water seems to never reach the bottom of the layer: it all evaporates or disappears into fissures eventually.
The main point of interest on the plane is the Teardrop Palace, a massive nautilus-shaped pagoda housing the largest black market in the universe: everything found here being stolen from the Great Wheel and beyond. Prices are high and the place is not without danger, but it sells a lot of exotic goods.
Hotter than the first layer, Chamada always flows with massive amounts of magma. The layer has very little in the form of usable surface: the ground always bursting open and erupting or being covered in lava and vision-blocking ashes.
On the slopes of Chamada stands the Tower Arcane: a great bladed and spiked structure holding the history of the Yugoloth race, written in the blood of flayed petitioners. Deeper still is the library of contracts: inscribed on the living skin of a petitionar and burned in with magic and branding irons. These people hang there in endless miles, with nothing on their minds but their pain and their contract.
Circling around Chamada is a small moon, roughly 2000 feet in diameter. This is Nimicri, a favorite trading post for those who travel the Planes. The moon's streets are clean, it's buildings well-maintained, and its inhabitants friendly. This of course has a darker side to it: Nimicri is actually a giant mimic copying a city. By giving it a drop of one's blood it can copy someone perfectly up to and including the point when it spilled its blood. Oh, and it sometimes eats visitors whole. So sleep well when you visit it!
The third layer is far less volcanically active than the first two: it is in fact quite cold and has snowfalls covering it. Acidic snowfalls. And the snow makes climbing the mountainside even more difficult. So have fun on Mungoth!
If a visitor should come upon the Valley of the Outcasts they should consider themselves lucky. Its inhabitants, the fire giant wizard Tastuo and her eight equally magically-inclined siblings, live there as outcasts and fugitives. Tastuo has made several contracts with the Yugoloth to ensure her safety if she were to be discovered by her non-defined enemies, her own plight making her sympathetic to those seeking refuge themselves.
The final layer of Gehenna, Krangath is dead. No volcanic eruptions, no wind, no light: nothing. It is dark and cold and empty. The few people on the layer will attempt to stay out of sight, in fear of Melif the Lich-Lord. Hopelorn, the citadel of Melif, is pretty much the only thing on the layer. An artifically maintained ledge holding a morturary city where necromantic energy lights the streets, it is a city of the dead. Melif and his fellow liches welcome their fellow undead but have no patience for the living. They often capture fiends for experimentation to unlock the secrets of life, death and being, but they are always careful to never capture Yugoloth, knowing that Melif's ass is grass if they do.
While originally from the Grey Wastes of Hades, the Yugoloth are the primary inhabitants of the plane, having emigrated en-masse because the Grey Wastes are, to put it politely, a shithole even by hell-plane standards. Other fiends also live here, mainly around the Teardrop Palace and The Crawling City. Because of its lava, malevolent Yugoloths and general dangerous nature, few mortals and other planars make their homes on the Bleak Eternity. Having said that, many Yugoloth work as mercenaries and gladly accept visitors willing to negotiate with them.