Home of the structured, evil devils, where they scheme and recruit mortal souls to their cause. Its inhabitants are as cruel as they are cunning, and the environment of every one of the nine layers of the Nine Hells is hostile to mortal life. The layers of the Nine Hells are as follows:
Avernus - The first layer of the nine hells and the primary battleground for the Blood War, the ancient war between the devils of the Hells, and the demons of The Abyss. It is ruled by the fallen celestial-turned-archdevil Zariel, who masterfully commands the devil hordes in her battlefield. It is said that Avernus was once at least pleasant-seeming, with temptations around every corner to lure in mortals, but also cities and centers of commerce for extraplanar trade. But the blood war destroyed pretty much all of that, except a few small areas. Avernus used to be ruled by several other entities, such as Tiamat and an archdevil called Bel.
Dis - Ruled by the archdevil Dispater, the Iron City of Dis encompasses most of this layer; so much so that to many people, Dis and the City of Dis are indistinguishable. Dis is a bustling devil city where the rules of hell are enforced extremely strictly. Good-hearted people should think twice about staying here for longer than necessary.
Minauros - Also known as the Endless Bog, Minauros is ruled by Mammon. Despite its putrid appearance, Minauros is a very wealthy place, due to its role in processing and minting new souls, but especially due to Mammon's business savvy and wealth as potentially the wealthiest individual across all the planes in terms of mere gold.
Phlegethos - Probably the most hellish of the places in the nine hells, ironically enough. Ruled by the Father-Daughter dream team of Belial and Fierna, Phlegethos is a realm of magma, unbearable heat, fire that seems to leap at interlopers, damnation and punishment.
Stygia - Hell froze over and gave us Stygia. A frozen ocean with murky, deep waters that is spotted with frigid islands, Stygia is both the prison as well as the kingdom of the archdevil Levistus, who got sealed away here after betraying Asmodeus himself. Nonetheless, the frozen master of Stygia has, after showing his prowess, been allowed to keep much of his power, and despite acting loyal to Asmodeus, is still desiring revenge. This is made harder by the archdevil Geryon, the former archdevil of Stygia, who is still trying to regain control of the layer by warring with Levistus and his forces. The river stix flows through Stygia's ocean and, interestingly enough, can maintain its integrity just fine.
Malbolge - Ruled by Asmodeus' own daughter Glasya, Malbolge is the layer of hell where devils are punished, when it's found out that they have violated the laws of the hells. Also known as the Crushing Lands, Malbolge is extremely inhospitable and unpopular even amongst devils, who greatly dislike the rocky avalanches that this layer is known for. The entire layer is on a slope, and finding any horizontal ground is impossibly, making even short rests difficult to do. Malbolge was once ruled by an Archdevil called Moloch, who has since lost much of his power and influence and now wanders the planes in order to try to raise an army to retake control of his former realm.
Maladomini - Ruled by the Archdevil Baalzebul, a former celestial from Mount Celestia who fell after doing unspeakable things in order to attain perfection. Maladomini, also known as the Circle of Ruins, is the center of the infernal bureaucracy. Crumbling cities that may once have been great can be found all over its surface; testaments to Baalzebul's sloth. Underground, one can find storehouses of infernal documents important to the hells.
Cania - Another frozen layer, not unlike Stygia but a hundred times worse, Cania is constantly shifting and dominated by snow storms, and much much colder than Stygia. Nonetheless, the archdevil in charge of this layer, Mephistopheles, actually keeps scattered libraries and laboratories sheltered in citadels on his layer, due to his interest in magic as the archmage of the hells. Despite his history of plotting against his master, Mephistopheles nonetheless counts as one of Asmodeus' closest confidants, indicating a form of special relation between the two.
Nessus - Ruled by THE archdevil himself, the Lord of the Nine Hells, Asmodeus. Entry into Nessus is strictly regulated, and you can not just waltz into it like you please; only those with Asmodeus' permission can enter. Here, one can find the city of Malsheem, which is arguably the largest city in The Outer Planes and the place of Asmodeus' residence.