Nessus

Nessus is the deepest, bottommost of the Nine Hells: the ninth layer, where Asmodeus himself is said to dwell.

Nessus is poorly mapped and poorly understood, since, if you couldn't guess, few who enter it ever leave it. According to Abn-Azer, it is a dark, misty realm of gibbering voices, while Mordenkainen describes it as a wasteland of crags and canyons.

Somewhere in Nessus is located Malsheem, the fortress of Asmodeus.