Gehenna

Gehenna is, for lack of a better term, a planet, the sole planet in a system that was once a burning quaternary star system, now four brown dwarf stars that orbit one another in an inscrutable pattern, Gehenna trapped in their violent twisting through space. The gravity of these four near-at hand, which Gehenna flies through and around, pulls the surface of the planet in directions that make its surface far from spherical. Indeed, some of these surface elements have ripped off of the planet entirely and now swoop through the atmosphere at alarming speeds and low altitudes. The gravity of the brown dwarfs (not visible through the smoke-filled atmosphere on the planet's surface) creates massive amounts of friction within the planet itself, which makes it incredibly hot in spite of the lack of any bright sun. Volcanic activity is intense, with massive cones erupting jets of flame, lava, and smoke high into the sky, sometimes hitting the jagged moon-structures as they pass by (and sometimes steady streams of lava form rivers of fire and rock that connect the surface of Gehenna to these satellites).    For unknown aeons, Gehenna was a place no one in particular cared much about. Its surface is entirely uninhabitable to most forms of life, and apart from some extremophile microscopic organisms and a few breeds of flame-fed lichen-like "plants," there are no life forms of note here. However, in somewhat more recent history, Gehenna has become perhaps the single most important strategic location within all of the Prime Material Plane. Early in the endless stalemate that is the Fiendstrife, the Demon Princes of the Abyss wished to create a concrete anchor in the Prime Material Plane where they might create a bridge (a crude but effective Krasnikov tube) of fluid that pours out of the chaos of the Abyss itself and into our realm, so that they might focus their invasion on the homeworlds of Devilkind, the Nine Hells. Gehenna, being the closest bit of solid ground to the Quyosh system (The Nine Hells), was where they chose to bore their hole in reality and stage their constant invasion. From deep within the volcanic lava-tube caverns of Gehenna the Styx flows out of pure void, rushing up to the surface, then twisting and writhing off into space, hurtling toward the Hells.    Asmodeus has therefore concentrated his front lines on Gehenna itself, while Avernus, the outermost planet of the Nine Hells, mostly serves as a staging ground for the devilish forces. A near constant flow of demons pours out of the void, making the interior caves and jagged surface of the planet a perpetual battleground between demons and devils. Occasionally, the demons are rallied by one or more of the Princes, and make a more organized (or at least, more savage) attack, wiping out the devilish forces on Gehenna and, by following the banks of the Styx through space, bring the assault to Avernus itself. Likewise, sometimes a general or Archdevil of the Hells makes a show of its prowess and devotion to Asmodeus (in hopes of attaining a higher position within the hierarchy of To'qqiz Shar) by leading an assault through the Styx-Gate on Gehenna into the Grey Wastes, and indeed, sometimes along the Styx through Carceri and into the madness of the Abyss itself, but invariably the utter chaos and violence of that place overwhelms any invasion by the devils, and thus neither side makes any headway in the Fiendstrife for very long.    With most of the fighting concentrated on Gehenna itself, the area is ripe for exploitation by unscrupulous creatures with a mercenary bent. There is no greater example of this in all Anuwai than the Yugoloths. Considered fiends likened to devils and demons by the post-human races, the Yugoloths are actually a federation of unrelated beings hailing from the Grey Waste of Hades. The yugoloths can be thought of as the "endgame" of corporate greed and ultimate, self-serving evil. After utterly obliterating all life on their own planet, they began to colonize and devour all life and resources they came across (these worlds collectively known as the Tarterian Depths of Carceri). While they busied themselves pillaging everything of value there (not in a viking-style organized chaos sort of sense, but in a strict corporate sort of structure designed by those at the top to be as efficient as possible at extracting anything worthwhile out of any given area), the Fiendstrife began raging, and the demons spilt the Styx forth through Carceri, onto and beyond the Grey Wastes, onto Gehenna, then plunging into the Nine Hell themselves. When the perpetual violence of the war between devils and demons began on Gehenna in earnest, the yugoloths (who'd nearly emptied Carceri) saw a new source of revenue. Their endless lust for flesh to be consumed could come from the bodies of both fiends slain in battle and of captured prisoners, while material wealth began flowing into both the Abyss and the Hells via their various off-world cults they had established. Devils started these cults primarily to attain new evil souls to bolster their forces, but were happy to bring in gems and gold as well so that they might pay off the new and willing yugoloth mercenaries on Gehenna. Demons, on the other hand, primarily started cults simply to spread their chaotic agendas of destruction across Anuwai, but like the devils were more than happy to milk them of any wealth they might attain in order to likewise buy off yugoloths.   The role of the yugoloths in the Fiendstrife is diverse; those lower on the corporate ladder are directed by their superiors to simply serve as foot soldiers in the war (and are happy to do it for the chance to plunder and loot all those they slay), while those higher up the corporate ladder are paid in mountains of gold and gems for their services as information and intelligence brokers. The seat of yugoloth power on Gehenna (and indeed, throughout the Prime Material Plane, as they have largely abandoned Carceri and even their homeworld of the Grey Wastes, which is now such a dismal place that despair and madness are all that can really survive there for very long) is The Crawling City, which is literally built on the back of a gigantic, rock-eating creature that resembles a house centipede (it's rumored to have originated in Carceri). On the back of this monstrous being is an elaborate, intricate city, which can move across the surface of the planet to keep away from the most violent battles. The yugoloths and the creature (called The Devourer) live in symbiosis; it provides protection for them by moving away from violence of its own accord, and they provide growth for it by constantly building structures on its carapace, which get added to the creature's own circulatory system and become part of its hardened shell. The top part of the shell is covered in towers, palaces, and treasuries holding the vast fortunes of the yugoloths. Merchants from elsewhere are happy to come here to sell the finest delicacies and trinkets to the opulent and selfish upper crust of yugoloth society. On the underside of the carapace hang stony barracks housing the footsoldier mercenaries at the bottom of the corporate ladder, as well as vast slaughterhouses that process the bodies of slain fiends to feed the mercenary armies of the yugoloths. While the devils are wed to the concept of order to such a degree that they would do great violence to impose it across all Anuwai, and the demons are so infused with violent chaos that they would, variously, destroy all life, maintain all life for bloodsport, reforge all life as unlife, or to end existence entirely, the yugoloths are a more calculating, sociopathic sort of evil. They empathize with no one but themselves, and to that end have boundless cruelty toward all others.   And so The Crawling City marches across the horrific battlefields of Gehenna, clearing away the bodies of the slain, while legions of yugoloth sellswords issue forth to whichever army has paid the most coin. The tide of battle constantly shifts and fluctuates, making Gehenna a truly awful place for anyone who values their life to be.