Nessus

Nessus was the ninth and deepest layer of the Nine Hells of Baator.

Geography
The layer was a featureless plain with jagged edges that extended from east to west and  from north to south, floating in an endless red void. The plains were scarred by chasms and ravines several thousand miles deep, and were frequently ravaged by fiery storm winds.

Geographical Features
The River Styx trickled down into Nessus through a little-known, small offshoot. It was heavily guarded by devils who slew any unauthorized visitors on sight. The river drained into a shallow pool known as the Forgotten Lake, then continued at the bottom of some of the crevasses before percolating through the earth and continuing its course into Gehenna.

The River Lethe also flowed across Nessus, occasionally also falling into its trenches and ravines.

Government
The layer was ruled directly by Asmodeus, who presented himself as an illusion of a tall, red-skinned, slender man. He was assisted by a court of archdevils that included the Chancellor of Hell Adramalech, Inquisitor Phongor, and several commanders. His pit fiend generals included Executioner Alastor, Asmodeus's majordomo Baalberith, and Constable Martinet.

History
Asmodeus allowed Zariel to recover in Nessus from the wounds sustained by her fall before assigning her as Duchess of Avernus.

Rumors & Legends
The tiefling midwife Destiny Agganor from Nightstone had a set of runes burned onto her front door to ward off unwanted visitors. It contained a prayer that included damning any intruders to burn in Nessus for almost a century.

Notable Locations

 * Malsheem, the largest city in all of the Outer Planes and home to Asmodeus. Located at the bottom of an enormous trench directly below the portal from the Pit in Cania, it was inhabited by millions of devils held in reserve for an unknown cataclysmic battle. Asmodeus's citadel of Fortress Nessus rose far above the plain from the bottom of the rift.
 * The Serpent's Coil, the deepest rift of the layer, said to have been blasted as a result of Asmodeus's fall into the Hells. It was home to his true form as he healed from the wounds sustained by his fall. The blood shed by Asmodeus's wounds constantly sprouted new devils, perfect specimens of pit fiends and cornugons.
 * Tabjari, a citadel made of copper that stood along the walls of Reaper's Canyon. It contained one of the three copies of the Pact Primeval―the other two remaining in Mechanus and Mount Celestia.

Inhabitants
Nessus was inhabited by a majority of greater devils, including Asmodeus's legions of amnizus, pit fiends, and cornugons, among others.

Appearances

 * Adventures
 * Storm King's Thunder (mentioned only)