The Nine Hells
The Nine Hells of Baator, sometimes shortened to Hell or Hells, and named Baator in Infernal, is the home of the devils and the plane that embodies lawful evil. It is a plane of sinister wickedness and institutional cruelty, its denizens organized into a strict caste system with a very rigid chain of command. Each of the nine Hells has its own physical laws or properties of matter, but all are inhospitable or deadly to outsiders. Malgathar, the evil god of War resides in his realm, the Brimstone Foundry, which is also located within Baator.
Each Hell is a different infinite layer interconnected at barriers much like a nine-layered cake—the lowest points of one layer manifest barriers that exit high above the surface of the next lower layer. The river Styx flows through the first layer, Avernus, and also the fifth layer, Stygia. Each of the Nine Hells is unique and usually mirrors the malevolent characteristics of its ruler, or perhaps the archdevils were are by the domains they schemed to control, no one can be certain.
The principal inhabitants of the Nine Hells/Baator are the devils, and their offspring in varieties too numerous to catalog here. Unlike the demons of the Abyss, the devils were highly organized in their quest for power and status—scheming and plotting power plays, coups, and assassinations. In addition to the devils, this plane is home to bonespears, gathra, haraknin, hell hounds, imps, night hags, nightmares, and maelephants. Also occasionally encountered are achaierai, barghests, hellcats, mephits, rakshasa, and stench kows. The nine circles of Hell are each ruled by an archdevil of great power, but the Nine Hells are also the home of other powerful beings.
Avernus , The Scorched Gate
The first circle of Hell, Avernus, is also the "topmost" because Astral travelers emerge from color pools on this layer and reaching the next circle requires descending to the lower depths to breach a barrier to Dis. Travelers on the Astral Sea who do not follow the Styx likely find themselves falling out of the sky above Avernus to a fiery death. 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 due to balls of flammable gas that float about or streaked across the atmosphere, randomly exploding as a fireball.
Dis, The Iron Despotate
The second circle of Hell, Dis, 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 center 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.The walls of the buildings and the stones of the streets glow the dull red of hot iron; more than brief skin contact results in severe burns. Prisoners of war, tormented underlings, criminals, and kidnap victims are kept in underground dungeons where their wails of woe can be heard filtering up through small vents in the iron walls. Above it all rises the Iron Tower where Dispater sits and schemes, untouchable.
Minauros, The Sinking Mire
The third layer, Minauros, 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 very difficult and is only broken occasionally by serpentine ridges of volcanic rock.Nameless creatures even the devils fear inhabit the swamp. Minauros as a realm is depicted as a broad but low-vaulted cavern connected to Dis. An oily water percolated through the roof of the cave and rained down upon swamps, deserts of mud and oozing black soil, pockmarked by bubbling fumaroles and mud geysers. Minauros is also the name of the city built of black stone by Mammon on the treacherous surface of this place. Only the ceaseless efforts of thousands of minions and slaves prevent the city from sinking and being consumed by the bog. The city of Jangling Hiter, also known as the City of Chains, hung by massive links of chain above the noisome fen and is ruled by kytons.
Phlegethos, The Furnace Womb
The fourth circle, Phlegethos, 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 and thus Phlegethos is considered to be fire-dominant. 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 provides visitors some protection from the elemental environment found throughout the rest of the plane.
Stygia, The Drowned Silence
The complete opposite of Phlegethos, Stygia, the fifth circle, is either a bottomless ocean covered by an ice sheet up to 3 miles thick. The river Styx cuts across the ice forming a channel. 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 strikes, a strange phenomenon called "cold fire" erupts: white flames of extreme cold that "burns" for a short time and then disappears without a trace. The great city of Tantlin is built upon one of these islands, 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 Shattered Descent
The sixth circle of Hell, Malbolge, is a gargantuan tumble of angular black stone blocks, each block ranging in size from a small city to a large metropolis, that forms 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 vapor rise up from the depths and light the sky with the color of blood, causing cosmologists to speculate that the blocks of Malbolge may have rested on an infinite sea of lava. Most habitations in Malbolge are copper-clad fortresses built from black stone.
Maladomini, The Wasting Reign
The seventh circle of Hell, Maladomini, is described as having vapor-polluted skies similar to Malbolge but the surface is solid. Maladomini is described as a colossal maze of passages each several miles across that eventually lead to 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 erupt from the ground. The Lord of the Seventh is never satisfied with the construction of his capitol and repeatedly builds and abandons city after city. The largest and most beautiful is Malagard, a sprawling metropolis/palace/fortress/arcology with myriad black towers linked by a tangled web of bridges and walkways. Malagard is rumored to contain a million rooms and to cap an equally complex dungeon labyrinth.
Cania, The Glacial Tyranny
Cania, the eighth layer of Hell, is a bitterly cold-dominant realm of solid ice mountains, titanic, unnaturally fast-moving glaciers, and nearly continuous snowfall that makes Stygia seem balmy by comparison. Unprotected travelers are exposed to temperatures of −60 ℉ but on the positive side there are few creatures that hunt in the icy wastes. Earlier lore describes the great citadel Mephistar as being constructed of iron but later reports say the Lord of the Eighth's fortress is made of ice. All accounts seem to agree the tower has a heated, luxurious interior and sat atop a gargantuan glacier called Nargus whose speed and movement are under the control of Mephistopheles himself.
Nessus, The Deepest Throne
The ninth and deepest Hell, Nessus, is a land of extremes: regions cold as Cania, volcanoes like Phlegethos, a lake of ice, a flaming forest, sheer cliffs, and firewinds. It is said that Malsheem, the Citadel of Hell, 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. A progression of rifts, pits, and chasms lead down and down, forming a vertical maze hundreds of miles deep that contains great cities, fiendish armies, and the mighty fortress of the Overlord Asmodeus.