The Nine Hells of Baator is an Outer Plane, and the birthplace of devils. Filled with smoke, fire, and ash, each Hell save the first is buried deeper underground. These continent-sized caverns contain fiery infernos, frigid wastelands, poisonous mires, and cruel fortresses, all of which are terrible beyond understanding. Only the greatest - and most foolhardy - adventurers dare to venture into this plane.
The Nine Hells
Baator is divided up into nine descending layers, each contained within a massive cavern beneath the layer above it. Each layer is ruled by an Archdevil, a powerful devil akin to a demigod. There is a strict heirarchy between them, with each layer being subordinate to the one below it.
The surface layer of Baator, Avernus is a blood-soaked rocky wasteland. Jagged mountains make navigation difficult, while fireballs rain from the sky. Avernus has long been one of the primary battlefields of the Blood War between devils and demons, and the remains of innumerable armies coats the landscape. Through the center of the realm runs the River Styx, whose waters rob all who touch it of their memories. Avernus has been alternately ruled by the pit fiend Bel and the fallen angel Zariel, who dwell in an imposing fortress known as the Bronze Citadel. Both have ever been fierce generals dedicated to winning the Blood War at all costs.
Beneath Avernus lies the city of Dis, a veritable maze of narrow alleyways between buildings of black iron whose layout constantly shifts and changes. At its center is the Iron Tower, from where the archdevil Dispater oversees mining and smelting operations that produce countless infernal weapons for him to sell to the highest bidder.
An oil rain perpetually falls upon Minauros, a massive, festering swamp whose foulness assaults all the senses. Much of the layer is dominated by the Sinking City, which is constantly built up even as it sinks inexorably into the mire beneath it. It is here that the souls of mortals are bartered and traded and various Batirian dukes vie for the favor of the miserly Mammon, the serpentine archdevil who controls it all.
The fiery cavern of Phlegethos is a constant inferno of heat and flame. The air is a choking cloud of noxious fumes, the rivers and lakes are boiling lava, and jets of fire burst from countless fissures in the volcanic rock. Those mortals who have survived the journey describe the fire as not being truly random, but rather actively seeking them out. The blasted landscape is dominated by the palace ruled by Lady Fierna and her father, Belial, who adjudicate the complicated laws governing the various contracts and agreements originating in Baator and administer what passes for justice in Hell.
Standing in stark contrast to Phlegethos, the realm of Stygia is a dark and frigid sea filled with titanic ice floes. The River Styx flows into these salty waters, yet is visibly distinct as it cuts through the cities that dot the largest icebergs. Stygia has long been a battleground between two competing archdevils, Geryon and Levistus. While Levistus eventually emerged the victor thanks to intercession from Asmodeus, Geryon's minions still battle those of Levistus and devils from throughout Baator join in Stygia's battles to sharpen their blades for the Blood War or settle scores with rivals.
For ages, Malbolge was an impossibly steep mountain slope covered with tumbling boulders, ruled not by an archdevil but by a night hag known as the Hag Countess. Recently, however, the Hag Countess was slain by Asmodeus's daughter, Glasya, who magically enlarged and reshaped the Hag Countess's body until it not only dominated Malbolge, but became the terrain itself. Glasya now rules Malbolge from within the Hag Countess's massive, misshapen skull, serving as the prison warden for those guilty of breaking the laws of the Nine Hells.
A realm of endless decay, Maladomini is filled with crumbling cities, sludge-choked rivers, dying forests, and strip mines. The sluglike Baalzebul rules this dominion, keeping watch over Hell's archive of edicts, policies, and deals.
Cania is a frozen wasteland of miles-high glaciers grinding against impossibly tall mountains, buffeted by furious blizzards. Beneath the snow lie the corpses of countless mortals who believed they could brave the chill of Cania. Cania holds few denizens beyond its legions of ice devils, and is ruled over by the silver-tongued Mephistopheles.
The innermost layer of Baator, Nessus is a blasted plain thousands of miles across floating in a void of swirling infernal vapors. Unlike the preceding layers Nessus is utterly flat, devoid of trees, mountains, or structures to obstruct the view. Beneath the surface, however, is a sprawling network of chasms, gorges, fissures, and tunnels within which lurk millions of devils of all sorts, from the lowliest imp to frighteningly powerful pit fiends. The Lord of Nessus is none other than Asmodeus himself, ruler of all the Nine Hells.