Nine Hells (/beɪˈɑːtɔːr/ bay-AH-tor)

The Nine Hells of Baator, sometimes shortened to Hell or Hells, and named Baator in Infernal, was the home of the devils and the plane that embodied lawful evil. It was 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 had its own physical laws or properties of matter, but all were inhospitable or deadly to outsiders.

Cosmology

The Great Wheel cosmology model placed the Nine Hells in the Outer Planes between Gehenna and Acheron, with additional connections to Concordant Opposition and the Astral Plane. Each Hell was a different infinite layer interconnected at barriers much like a nine-layered cake—the lowest points of one layer manifested barriers that exited high above the surface of the next lower layer. The river Styx flowed through the first layer, Avernus, and also the fifth layer, Stygia, before crossing over into Gehenna.
When the Great Wheel model was overshadowed by the World Tree cosmology model, the river Styx was renamed the River of Blood and it flowed through all the fiendish planes (except for the Supreme Throne and the Demonweb Pits) originating in the Abyss, passing through the Blood Rift—an unusual plane that connected the Abyss with the Nine Hells—bringing the devils even closer to their arch-enemies the demons, resulting in the Blood War. The layers were still described as being infinite but with a central pit of finite size that opened to the next lower layer in a tiered fashion, with a drop of many miles between layers. Cosmologists verified portals between the Nine Hells and the Barrens of Doom and Despair and Clangor and, by agreement with Kelemvor, to the Fugue Plane. The Astral plane connected all of the fiendish planes to the Prime Material Plane, but not directly to each other.
After the Spellplague, Asmodeus consumed the essence of the fallen Azuth achieving (some say regaining) greater godhood and ended the Blood War by casting the Abyss to the furthest depths of the Elemental Chaos. The World Axis cosmology model described the Nine Hells as an astral dominion floating in the Astral Sea, no longer of infinite size nor consisting of layers, ruled by Asmodeus and his eight archdevil vassals. Once again the river Styx flowed through the Nine Hells and the Abyss, but then emptied its pollution into the Astral Sea.