Nessus, the Ninth Hell.

The ninth and deepest Hell was a land of extremes in the Great Wheel view: regions cold as Cania, volcanoes like Phlegethos, a lake of ice, a flaming forest, sheer cliffs, firewinds, and a citadel even larger than Khin-Oin in Hades (later, the Khin-Oin became part of the Abyss)
The World Tree view did not contradict this description of Nessus but focused more on the blasted and torn landscape out of which rose Malsheem, the Citadel of Hell. It was said that Malsheem could hold millions of devils within its mountainous edifice, from the lowest warrens deep in the trench to the soaring spires miles above the tortured plane.
The World Axis model agreed that a progression of rifts, pits, and chasms lead down and down, forming a vertical maze hundreds of miles deep that contained great cities, fiendish armies, and the mighty fortress of the Overlord Asmodeus.[

Location under

Outer Plane - Nine Hells