Nessus

The lowest layer of the Nine Hells, Nessus is a realm of dark pits whose walls are set with fortresses. There, pit fiend generals loyal to Asmodeus garrison their diabolical legions and plot the conquest of the multiverse. At the centre of the layer stands a vast rift of unknown depth, out of which rises the great citadel-spire of Malsheem, home to Asmodeus and his infernal court.
Malsheem resembles a gigantic hollowed-out stalagmite. The citadel is also a prison for souls that Asmodeus has locked away for safekeeping. Convincing him to release even one of those souls comes at a steep price, and it is rumoured that the Archduke of Nessus has claimed whole kingdoms in the past for such favours.
Asmodeus most often appears as a handsome, bearded humanoid with small horns protruding from his forehead, piercing red eyes, and flowing robes. He can also assume other forms and is seldom seen without his ruby-tipped sceptre in hand. Asmodeus is the most cunning and well-mannered of archdevils. The ultimate evil he represents can be seen only when he wills it so, or if he forgets himself and flies into a rage. (https://www.dndbeyond.com/sources/dmg/creating-a-multiverse#OtherPlanes)

Alternative Name(s)

Ninth Layer

Type

Dimensional plane

Location under

The Nine Hells

Owner/Ruler

Asmodeus

Ruling/Owning Rank

Archdevil

Characters in Location

• Asmodeus