Nine Hells

The Nine Hells represent pain, suffering, never-ending torment, and also cruel forms of order and law. Across these nine layers sit the most wicked and black hearted tyrants in all the multiverse. Evil are law are the most appropriate words to describe the Nine Hells. The evil within the Nine Hells is one born of calculation and premeditation designed only to gather power through the use of contracts and souls.
Comparisons between the Nine Hells and the Abyss are wholly misguided, while they are both bastions of evil and can in some places have similar geographic features, the fundamental nature of the Nine Hells and the Abyss could not be more different. The Nine Hells are law and order incarnate with extensive bureaucratic and hierarchical systems, while the Abyss is chaos, destruction, and disorder given form.
Law and order are everything within the Nine Hells, they serve as a fundamental force felt and obeyed across all the layers. All devils follow rules and the law, they are inherently creatures of order, and order has a way of asserting itself upon plane.
Some say the order that permeates all of the Nine Hells is a manifestation of the power of Asmodeus, the Lord of Nessus, the ninth and lowest layer of Baator. He is the only devil lord to truly sit in a position of divine power. It is Asmodeus who wields more power than any entity within the Nine Hell and it is by his whim much of the political intrigue is pushed and arranged.
Devils are the most common inhabitants of the Nine Hells. They serve order and law, and and the Lords of the Nine by virtue of their rank, and they advance only by adhering to the laws to the letter through assassination, subterfuge, and forcing their superiors into positions of humiliation. A devil can rise quickly in the ranks, especially through the lower forms of devilkind, by advancing the campaigns of their dukes and Archdevils.
The conflict that defines the Nine Hells is not one with the mortals of the Prime Material Plane, but instead an incomprehensibly titanic and sprawling conflict known as the Blood War with orderless Abyss and demonkind. The War is one between order and chaos, law and anarchy which plays out across the Lower Plane and fronts upon practically every plane of existence. The War is truly cataclysmic and untold millions, perhaps more, devils and demons clash obliterating one another, and if devils and demons alike are to be believed the Blood War has led to the destruction of countless worlds. The few individuals that true understand the Blood War insist it must continue until eternity’s end, either because it is a fundamental conflict within existence, or because if either force won, then the forces of either the Abyss or Nine Hell would be free to exert their will on creation.
The greatest currency the devils of the Nine Hells have in this never-ending and all encompassing Blood War is souls. Devils capture souls and transform them into new devils, and so devilish agents are sent out across the multiverse to collect souls. The laws of the Nine Hells extend to bargains with these creatures so mortals that pledge their soul to a devil are signing up for an eternity of conflict, likely in the Blood War but perhaps as a pawn of a petty duke or lesser devilish lord.

Portals to the Plane

Of all the portals to the Nine Hells most lead to Avernus, the broken land domained by the Blood War. Many portals exist across the planes that lead there, with both spontaneous gates and other permanent portals present. When a new portal is discovered by the devil forces of Avernus, they move quickly to control it by building a fortress around it or disable it if it is at risk of falling to demons. These portals are capable of forming in places of great heat and cold, but there must always be suffering or bloody order.
There have been remarkably few portals recorded that have led to lower layers of the Nine Hells, so few are they that most scholars do not believe they are capable of existing. For the Nine Hells and its inhabitants there seems to be a barrier that inhibit their travel to other planes of existence, having to rely on being summoned or naturally occurring portals to travel to other planes, although they seem perfectly capable of transporting themselves back to the Nine Hells.

Lords of the Nine

Within the most powerful entities are the archdukes and archduchesses and greatest of them are the Lords of Nine, they sit at the top of bureaucracy and hierarchy of Nine Hells as its absolute rulers. Individually, these are powerful entities that have power that rivals that of the divine, which allows them to control and direct geographic and environmental forces nearly at will. The greatest among them is Asmodeus with powers that are truly divine, although even he knows that each of the other Lords of Nine are almost equal in power to him.

Avernus, First Layer of the Nine Hells

Zariel: The Lord of the First and commander of the diabolical armies in the Blood War, Zariel is a fallen angel who was appointed to the Lords of Nine by Asmodeus himself.

Dis, Second Layer of the Nine Hells

Dispater: The Lord of the Second, an archdevil whose extreme vigilance almost personified caution. He is believed to be amongst the greatest arms dealers in all the planes, is perhaps one of the most knowledgeable creatures in existence, and one of the oldest Lords of Nine.

Minauros, Third Layer of the Nine Hells

Mammon: The foremost merchant and miser of the Nine Hells, and perhaps the richest entity in all the planes. As the lord of Minauros, Mammon oversees the vast and expansive soul trade.

Phlegethos, Fourth Layer of the Nine Hells

Fierna: The official Lord of the Fourth. Fierna is a creature of almost unrivalled charisma, she is a brilliant manipulator capable of filling mortal and immortal hearts with whatever emotion she chooses to evoke.
Belial: The predecessor to Fierna who also lays claim to the title of Lord of the Fourth, Belial is in part the power behind the throne of Phlegethos, dealing with the minutiae of the actual duties of ruling.

Stygia, Fifth Layer of the Nine Hells

Levistus: The Lord of the Fifth is currently imprisoned in an iceberg by Asmodeus for past crimes. He is therefore forced to telepathically relay instructions to his minions, even as he also has to deal with his predecessor's continued existence.
Geryon: The deposed Lord of the Fifth, Geryon is a savage archdevil who seeks to reclaim his former position.

Malbolge, Sixth Layer of the Nine Hells

Glasya: Lord of the Sixth, the rebellious daughter of Asmodeus, rules the place and oversees the punishments doled out to devils that stray from their assigned tasks.

Maladomini, Seventh Layer of the Nine Hells

Baalzebul: Lord of the Seventh, and one time ruler of two Layers before plotting against Asmodeus. Baalzebul oversees the bureaucracy of the Nine Hells and its organisation throughout the Planes.

Cania, Eighth Layer of the Nine Hells

Mephistopheles: Lord of the Eighth, is the foremost wizard in the Hells, Mephistopheles suffers nothing that would compromise his intellectual focus. He is also the Lord of Nine who maintains the security of Layers.

Nessus, Ninth Layer of the Nine Hells

Asmodeus: The Lord of the Ninth and King of the Hells, Asmodeus is the ruler and most powerful creature in all of the Nine Hells; his power is truly equal to that of the deities themselves.

Denizens of the Hells

Devils are the most commonly encountered inhabitant of the Nine Hells, but each of the layers have more monsters to offer unwary travellers than most expect. However within the Hells other creatures are found yugoloth, oozes, and unfortunate humanoids.