The vast and gloomy ninth and nethermost plane of the hells is known as Nessus, or Cocytus (after the lake therein), and is the personal realm of Asmodeus, Overlord of the hells. Asmodeus inhabits a grand palace, a fey court of darksome beauty, which rests upon the floor of the lowest rift in Nessus. If this abode has a name, it is little used and unknown to men. It is known that smoke rises from its very stones when they are trod by one not of lawful evil alignment, and that Asmodeus has gathered here all objects that he finds beautiful (including much seen as beautiful by man).
It is known that Asmodeus can summon each arch-devil to his court here, once a year, but more details of his palace are few indeed, understandably, since few travelers return whole to the Prime Material Plane after being in the clutches of the Great Devil himself. All manner of beings may be found within its extensive halls, for Asmodeus, a master strategist, makes use of all the powers and talents available in the hells to further his own ends. His bodyguards and personal servants are the mighty pit fiends; outside of the palace, one is more likely to encounter them in Nessus than any other creature, for they are constantly flying here and there at the Overlord’s bidding.
Knowledge of the geography of Nessus is similarly incomplete and possibly inaccurate, but it is known to be a rocky, misty realm that holds many dangers for the unwary.
Lethe, the river of forgetfulness, winds aimlessly about Nessus; no mortal has ever found its source or outflow. All creatures who touch its clear green waters (except devils, who are immune to Lethe’s power) must save vs. poison or be permanently feebleminded (only a wish, alter reality, or limited wish will reverse this), and even if cured, victims will lose all memories of the time preceding their cure. Some say that the river Lethe flows through dimensional space in some incomprehensible way, and thereby also flows on the Prime Material Plane. It may thus provide another entry and exit to the deepest plane of the hells – but if any have traveled this route and survived, they do not talk of it.
The lake of Cocytus lies far from the palace of the Overlord, high up in a bowl of rock surrounded by crags. It is frozen; a glassy-smooth surface of ice tens of feet thick – and within the ice are trapped the bodies of all who have ever ventured across it, for the ice always rises suddenly, in jaws, to engulf them. A dispel magic or slow will slow this action enough for a creature to escape or be helped clear, in haste, and a wall of force or even a wall of ice will temporarily jam the jaws open and also permit escape. A shatter spell the physical force wielded by most creatures will not affect the ice, but a crystal brittle spell cast upon it will render the ice fragile (to the extent of the spell’s area of effect), subject to the effects of all normal weapons and implements, and a hammer of thunderbolts will shatter it easily (perhaps also damaging a trapped victim). Any magical and/or natural combination of strength equal to a storm giant’s (such as the natural might of Asmodeus or Geryon) can affect the ice of Cocytus.
Within the course of the river Lethe is the central area of Nessus, a region of twisted and scorched rock broken by many rifts and knife-edged ridges, and within this area the firewinds sweep. A firewind is a permanent wall of fire (as in the spell) of triple strength and damage, typically up to a quarter of a mile in length, that sweeps over the ground in the forefront of a howling wind (that can inhibit movement). The firewinds blow at random, and may join or break apart in accordance with the topography of the land or for no apparent reason. Usually they will sweep over a creature on the ground in a single round, enveloping it briefly for 4-24 points of damage (save for half damage). Loose garments, small birds, and so forth may be blown away with the firewind, but in its wake there is relative calm. The bones of many creatures litter this inner desolation of Nessus, and a traveler will find that the rifts grow deeper as one heads inward, so that the bottom of the lowest plane of the hells is a huge bowl or gulf. Here the air is always smoky, and the firewinds howl, and here, at the bottom of the lowest rift (where the firewinds do not reach, but pass overhead) is the infernally grand palace of Asmodeus. Some sages say that in the center of the palace is a great gate that will allow transport from this place to any plane (of the user’s choice) in the multiverse. This gate, they say, will allow passage of any number or amount of creatures and objects, and Asmodeus cannot destroy it or affect its operation. (Devils and other creatures of lawful evil alignment attempting to use it, regardless of power, will be destroyed.) Some believe that the Overlord deliberately built his palace atop it so as to control access to and from the hells; somewhere in the Prime Material Plane, it is hinted, lies the way into the Nine Hells via this gate. The existence of this gate is supported by the legend that to escape the hells, one must descend to the deepest part of the plane. The maker of the gate is unknown, but is probably one or more of the greater gods.
Asmodeus rarely leaves his palace, making his will known and acted upon by servants of apparently unshakeable loyalty. Adramalech serves him as chancellor, recording the numbers, acquisitions, and transformations of the lemures and larvae, torments suffered by devils, the names of those who serve the diabolic on other planes, and the making and terms of all pacts and contracts. The Keeper of the Records answers to no one but the Overlord himself, and has his own tower in the palace, ringed at all times by six pit fiends. This guard detail is ordered by the pit fiend Baalberith, major domo of the palace, probably at the behest of Asmodeus, and is little liked by Adramalech. A similar guard rings the chamber or person of Bensozia, consort to Asmodeus, at all times. Another 54 pit fiends round out the palace guard. Phongor is the most feared devil in Nessus, after Asmodeus himself; he serves Asmodeus as inquisitor, and little information escapes his probing questions and methods of persuasion.
The greatest pit fiend of all, the awesomely scarred, broken-winged Alastor, serves Asmodeus as executioner. His strength is that of a storm giant’s, and he never speaks. It is said in the hells that if all were swept away, and Asmodeus could choose but one devil as a companion, that one would be neither consort nor lieutenant, but Alastor the Grim.
Another pit fiend, Martinet, is constable of Nessus, responsible for the personal armies of Asmodeus. These armies camp eternally around the palace, and their generals confer often with the Overlord. These proud dukes are: Buer, who commands 15 companies of pit fiends; Morax, leader of nine such companies; Bune, general of 30 companies of malebranche; Rimmon, head of five companies of ice devils; and Zagum, leader of 30 companies of barbed devils. It is said that the nycadaemon, Daerith, once appeared unannounced over the lake of Cocytus and flew toward the palace of Asmodeus, and before it reached the crags about the lake was destroyed utterly, torn apart bodily by pit fiends on patrol. With such guardians, it is small wonder why the mysteries of the plane are so many, and the visitors who return whence they came so few.