Nessus is the ninth and deepest layer of Baator. It is a place of pits and ravines of virtually endless depths. The great citadel of Malsheem lies immediately below the portal between Cania and Nessus and is the largest fortress in all of the Outer Planes. Here, Asmodeus rules over the entire plane, and thus the entire race of devils.
The surface of Nessus is a plain shattered by rifts deeper than the deepest ocean trench. Many of the ravines and canyons here reach thousands of miles into undifferentiated, dead stone. Most of the trenches seem natural, but some appear as if they were cut or blasted into the land. Rumor has it that an offshoot of the River Styx flows here and there, dropping into trenches and trickling its way across the layer. Few know how to reach this tributary, if it exists at all.
A rift of incredible depth and width lies immediately below the layer boundary between Cania and Nessus. Malsheem, the Citadel of Hell, rises in its dark, elegant, fiendish beauty from the trench. From here, the Overlord of Hell, Asmodeus, rules the plane of Baator.