Nessus is the ninth and almost deepest layer of Baator, however this is the last realm that devils may roam freely. 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 whom rules over the entire plane, and thus the entire race of devils.
However below this plane is the 10th and final plane known as the Forbidden Wasteland of Pandemonium the Final Plane of Hell.
The surface is currently unknown.