Atlea and Beyond
The Nine Hells of Baator are nine layers of a planar dominion in the Astral Sea ruled by devils. Asmodeus, the god of tyranny, is the true ruler of Baator, but he has many underlings. Each layer of Baator has an archdevil that rules it and everyone living there.
The layers and archdevils are as follows:
Avernus, ruled by Bel (and the Dark Eight).
Dis, ruled by Dispater.
Minauros, ruled by Mammon.
Phlegethos, ruled by Fierna.
Stygia, ruled by Levistus.
Malbolge, ruled by Glasya.
Maladomini, ruled by Baalzebul.
Cania, ruled by Mephistopheles.
Nessus, ruled by Asmodeus.