The Nine Hells of Baator, the infamous home of devils in Dungeons & Dragons, is a place of pure evil and despair. Devils follow a rigid, militaristic system, living under the tyrannical rule of Archdevil, Asmodeus.
Those Asmodeus has deemed most worthy are promoted to Archdevils, like himself. He has entrusted each of them with ruling one of the nine layers which they bend to their will. These nine hells are an iconic aspect of Dungeons & Dragons, each with their own unique style and lore.
AVERNUS
No planar portals can reach any other layer of the Nine Hells, so Avernus acts as the arrival point of all outsiders. It is a desolate, rocky wasteland, broken by rivers of blood, and infested with swarms of biting flies. The fiery skies rain down comets that leave smoldering craters in their wake. It is also the primary battlefield for the Blood War between devils and demons. As such, there are remains of old battles as far as the eye can see, ancient bones clinging to rusted weapons and crumbling embattlements.
Zariel rules the realm. Once an angel assigned to observe the Blood War for the celestials, she soon became obsessed with the struggle. Though told not to interfere, she was convinced she could beat the devils. She launched an attack on Avernus, bringing mortal warriors with her. They were eventually overwhelmed and killed. But Asmodeus had her reborn and appointed the realm of Avernus for her to rule, offering her the eternal battle she craves.
DIS
The second layer of the Nine Hells is a winding maze of crags and canyons, rich with iron ore. All roads in the plane of Dis are made entirely of iron guarded by fortresses built into the sides of jagged mountains. It's ruler, Dispater is the primary arms dealer in the Nine Hells. The ore from his layer is mined and crafted into the weapons which fuel the many legions.
Beings from across the planes come to the Iron City of Dis to broker deals with devils, hags, or other fiends within. Dispater is the most concerned with collecting souls and is known for making deals with mortals more than the other Archdevils. He is deeply paranoid, so his acquiring of a soul usually involves gaining some new secret to hold over his foes.
MINAUROS
Minarous is a foul, decrepit bog. The skies rain down acid and cavernous pits wait to devour the unwary. At its center rises the slimy walls of the Sinking City, home of Mammon, the ruler of Minauros. Mammon is the banker of the Nine Hells, miserly, and greedy. He is the only Archdevil to value coin as well as souls, and he hoards his wealth deep in his domain.
He refuses to invest in the infrastructure of Minauros, which is why his kingdom remains in shambles. He never rests, always seeking another way to grow his massive wealth. He is dangerous to deal with, as his mind is always looking for a way to take advantage. Every devil who harvests souls for Mammon is required to carry a copy of "The Accounting and Valuation of All Things," which allows devils to assess the value of a soul in gold or goods.
PHLEGETHOS
The fourth layer of Baator is ruled by rivals Belial and Fierna. It's a fiery hellscape of flowing lava, whirlwinds of foul smoke, and coated in ash. The two rule from their fortress in the caldera of the volcano, Abriymoch. Typically, there can be only one rule to a layer of hell, but Asmodeus allows these two to reign together in a semi-friendly competition.
Belial oversees the Diabolical Court, the judiciary system of the Nine Hells. He is a terse, intimidating figure. Fierna, on the other hand, is a charming diplomat who is capable of bending mortals to her whim. Various claims have been made that they are husband and wife, brother and sister, father and daughter, or mother and son.
STYGIA
Stygia is a realm of bitter cold and freezing fire. The sky breaks with lightning, and a frozen ocean surrounds the layer. At the center, entombed in a prison of ice, rests its lord, Levistus. He was punished by Asmodeus long ago for reasons only the two know. With nothing but time on his hands, Levistus listens to the mortals who reach out to him and seeks to turn them into his agents.
It is unique in that all the other layers serve a specific function for Hell's war machine, but Stygia is trackless wastes. It is also home to many dangerous creatures from the material plane, leading some to believe it wasn't always part of the Nine Hells. Devils come to Stygia to test themselves against its harsh environs and residents.
MALBOLGE
The sixth realm is an eternally sloping mountainside and serves as the prison and torture chambers of the Nine Hells. Those found guilty in Phlegethos are sent here for punishment. While there are structures built into the slope, boulders and landslides are a constant threat, wreaking havoc on the landscape.
It is ruled by Glasya, the rebellious daughter of Asmodeus. Unlike other demons, she flaunts tradition. While she won't outright break the rules, she will bend them. She is also responsible for creating the Nine Hell's first crime syndicate, the Coin Legion. Her crew counterfeits currency allowing her to turn an incredible profit when trading for souls. Her legal acumen has kept her from winding up in trouble with the other Archdevils, even after being caught red-handed.
MALADOMINI
The domain of Baalzebul houses all the records for the Nine Hells. Winding underground mazes contain every contract signed or deal struck, designed in such a way that only Baalzebul and his agents can find anything. It is also a pathetic stretch of ruined cityscape, rife with neglect and decay.
Baalzebul was transformed into a bloated degenerate monster as punishment by Asmodeus for trying to usurp him. This setback has not deterred him at all, despite failing at every turn. He is subject to a curse which causes any deal made with him to end in utter catastrophe. Asmodeus shows him favor from time to time, though it's not clear if it's out of pity or respect.
CANIA
Cania is a land of bitter cold, like Stygia. However, the eighth layer serves a much more critical function. Ruled by Mephistopheles, Cania is essentially one giant laboratory, with cities nestled in the ice designed to carry out arcane experiments. He specializes in luring the souls of spellcasters, hoping to bring them to his realm to further his research.
He is widely considered the foremost wizard in the Nine Hells. From his seat of power in Mephistar, the ruler of Cania plots to overthrow Asmodeus and claim the Throne of Baator for himself. Simultaneously his greatest rival and ally, Asmodeus, values his advice, knowing full well that Mephistopheles is waiting for him to make a mistake.
NESSUS
Asmodeus, the unmatched ruler of the Nine Hells, calls this barren domain his home. Devoid of activity by design, it is covered with darkened pits lined with fortresses housing his loyal pit fiend generals and their forces. He resides in the tower of Malsheem, a hollowed-out stalagmite jutting out of a vast rift in the realm. He is known to imprison souls in Malsheem that he deems worth keeping.
Charismatic and well-spoken, he has power beyond that of all other devils, enough that some mortals worship him as a god. Despite that, he rarely speaks directly to his followers, using emissaries and agents to convey his will. Long ago, he was put on trial by the gods for his atrocities. He justified the continued existence of devils, but as punishment, he is forced to uphold the rules of the Nine Hells strictly and cannot break a contract once signed without suffering repercussions. He carries with him a ruby scepter that binds him and all devils to this fate.