The Infernum

The Infernum, sometimes Hell or Nine Hells, also known as Baator in Infernal, is the home of the devils under the command of Balor, Arch-Devil. It is an elemental plane of sinister evil and institutional cruelty organized in a strict caste system with a very rigid chain of command. Unlike the daemons of the Void, the devils are highly organized in their quest for power and status—scheming and plotting power plays, coups, and assassinations. Each of the nine Hells has its own physical laws or properties of matter, but all are inhospitable or deadly to outsiders.
Souls destined for the Infernum arrived as mindless imps or, if they were worthy, as semi-intelligent Lemures. Souls first travel the Long Shadow to reach the Gates of Judgement, where they await escort to their final rest on the plane of their primary deity. While waiting, devils often bargained with the souls, playing on their fears and doubts to get them to agree that becoming a Lemure with a chance for promotion was a better option than their suspected fate. Strong or crafty souls might negotiate a deal that reduced their time as a Lemure or bestow a boon or punishment on those they left behind.

Geography

Each Hell is a different infinite layer interconnected at barriers much like a nine-layered cake—the lowest points of one layer manifested barriers that exited high above the surface of the next lower layer. The river Styx flows through the first layer, Avernus, and also the fifth layer, Stygia, before crossing over into the Ethereal Plane. Each of the nine Hells is unique and usually mirrors the malevolent characteristics of its ruler, or perhaps the devils were shaped by the domains they schemed to control, no one can be certain.
Avernus, the first layer
The first circle of Hell was also the "topmost" because travelers would emerge from colour pools on this layer and reaching the next circle required descending to the lower depths to breach a barrier to Dis. Theoretically it could also be reached from the Ethereal plane or the river Styx, however this would require foresight of the location of devils who patrol their borders in a strict and unnaturally efficient manner.
By all accounts Avernus is a desolate wasteland with rocky terrain, sparse, twisted vegetation, concealed snake pits, caves and warrens, volcanoes, and rivers of magma. The sky is starless, full of choking smoke, and glows a dark red hue due to the balls of flammable gas that floated about or streaked across the atmosphere, randomly exploding as a fireball. Due to its uppermost location it serves as a prime battlefield of the Sanguinem Belli: Avernus echoes with the marching of legions of devil troops preparing for the next campaign against the daemons of the Void, and the ground is littered with the detritus of countless battles.
Dis, the second layer
The second circle of the Infernum, when described as its own layer, is a flat barren plane containing little more than black, stagnant rivers, stretching for thousands of miles until it reaches some rolling hills. The sky is a cloudy dull green shot through with lightning. In the centre of this plane rises the Iron City of Dis, several miles in height and hundreds of miles wide. The foul rivers radiate from a moat big enough to be called a lake surrounding the Iron City.
Minauros, the third layer
Minauros as a layer is described as an endless bog of vile pollution, decaying bodies, and rotting marsh, repeatedly drenched by rain, sleet, and hail storms. The soggy, bone-strewn, disease-ridden swampland makes movement difficult and is only occasionally broken by serpentine ridges of volcanic rock. Nameless creatures even the devils fear inhabited this vile swamp. Minauros as a realm is depicted as a broad but low-vaulted cavern connected to Dis. An oily water percolates through the roof of the cave and rains down upon swamps, deserts of mud and oozing black soil, pockmarked by bubbling fumaroles and mud geysers.
Phlegethos, the fourth layer
The fourth circle is the Hell that most resembles the stereotype of a fiery world of eternal damnation, filled with active volcanoes, rivers of liquid fire, molten rock, ash hills, smoking pits, unbearable heat, all wracked by tremors and earthquakes. Even the air seems aflame here and thus Phlegethos was considered to be fire-dominant and likely has at least one connection to the Plane of Fire. Phlegethos as a layer is a cavern several mile s below Minauros, where burning lava pours out of fissures in the ceiling. The city of Abriymoch is the seat of power in this realm, built of hardened magma, obsidian, and crystal in the caldera of an extinct volcano which provide visitors some protection from the elemental environment found throughout the rest of the plane.
Stygia, the fifth layer
The complete opposite of Phlegethos, Stygia is either a bottomless ocean covered by an ice sheet up to three miles thick, with the river Styx cutting across the ice forming a channel. The River Styx also supports small but hardy plants and mosses which, after millennia of decay of this vegetation, resulted in swampy areas along the banks of the river. A few floating islands are the only non-frozen ground in Stygia, their peaks wreathed in lightning arcing from the coal-black sky. Where lightning struck, a strange phenomenon called "cold fire" erupts: white flames of extreme cold that "burned" for a short time and then disappeared without a trace. The great city of Tantlin was built upon one of these islands, in the curve of the swampy Styx, or perhaps on a giant ice floe. Due to the proximity of the Styx, Tantlin is a cross-planar trading post for those brave enough to attempt navigating the treacherous river.
Malbolge, the sixth layer
Malbolge is a gargantuan tumble of angular black stone blocks, each block ranging in size from a small city to a large metropolis, that form a pile hundreds of miles thick. The randomly tilted and ill-fitting blocks are honeycombed with angular passages and caverns causing non-flying travelers to frequently need mountaineering skills and risk avalanches. Stinking clouds of vapour rise up from the depths and light the sky with the colour of blood, causing cosmologists to speculate that the blocks of Malbolge may rest on an infinite sea of lava. Corroborating reports have been heard of flammable materials left on the ground spontaneously combusting. Most habitations in Malbolge are copper-clad fortresses built from black stone, where regiments upon regiments of Lucifers armies reside.
Maladomini, the seventh layer
The seventh circle of Hell is described as having vapor-polluted skies similar to Malbolge but with a solid surface. The erudite explorer Muldefix described Maladomini as a colossal maze of passages each several miles across that eventually lead to the layers of Cania, Malbolge, and Nessus. The seventh Hell is filled with ruins of old cities, stagnant rivers, exhausted and abandoned quarries and strip mines, stone aqueducts and lava canals, decaying fortresses, swarms of biting flies, and black pools of ichor that spontaneously erupt from the ground. The Lord of the Seventh was never satisfied with the construction of his capitol and repeatedly built and abandoned city after city. The largest and most beautiful was Malagard, a sprawling metropolis/palace/fortress/arcology with myriad black towers linked by a tangled web of bridges and walkways. Malagard was rumored to contain a million rooms and to house an equally complex dungeon labyrinth.
Cania, the eighth layer
Cania is a bitterly cold-dominant realm of solid ice mountains, titanic, unnaturally fast-moving glaciers, and nearly continuous snowfall that make the layer of Stygia seem balmy by comparison. Unprotected travelers are exposed to temperatures well below freezing point, but on the positive side there are few creatures that hunt in such icy wastes. The great citadel Mephistar was constructed here by the Lord of the Eighth as a fortress made of ice. All accounts seemed to agree that the tower has a heated, luxurious interior and sits atop a gargantuan glacier called Nargus whose speed and movement were under the control of Mephistopheles, Lord of the Eighth himself.
Nessus, the deepest layer
The ninth and deepest Hell is a land of extremes: regions cold as Cania, volcanoes like Phlegethos, a lake of ice, a flaming forest, sheer cliffs, firewinds, and a citadel even larger than Sigil itself. Out of the blasted and torn landscape rises Malsheem, the Citadel of Hell and seat of Balor himself. It is said that Malsheem can hold millions of devils within its mountainous edifice, from the lowest warrens deep in the trench to the soaring spires miles above the tortured plane. Here the Overlord of Hell, Balor, schemes and commands his near-infinite forces in the Sanguinem Belli.

Alternative Name(s)

Hell, Nine Hells, Baator