The Nine Hells

The Nine Hells is the outer plane of the lawful evil alignment, consisting of nine layers: Avernus, Dis, Minauros, Phlegethos, Stygia, Malbonge, Maladomini, Cania, and Nessus. The layers build into each other and are only accessible by neighboring layers; it is only possible to connect to the Prime Material Plane from Avernus, which cannot lead into Minauros without first passing through Dis, and so on. Nessus is the ninth layer and the home of Asmodeus, the supreme archdevil who rules the Hells.
There is no day or night in any of the layers, only a bloodred sky which rains fireballs on any non-fiend creatures that may be present. The geography is distinct from layer to layer but is almost invariably barren, rocky, and unbearably hot, riddled with ash and magma. Most layers are crawling with fiends, specifically devils, ordered in a coordinated society whose explicit goal is to spread evil and corruption.
Denizens of the Nine Hells make use of those in Hades to corral humanoids into the Abyss, where they are tortured for eternity. Thus the three evil-aligned planes share a symbiotic relationship.

Type

Dimensional plane

Alignment trait

Lawful Evil