Phlegethos is the fourth plane of the Nine Hells that’s ruled by Belial. It is a tortured realm of volcanoes, gouting fires, hills of ash, and pits of smoking dung. The ground is always uncomfortably hot – most intruders are constantly on the move – and well patrolled by groups of barbed devils. Ground tremors are common, and a sudden, violent eruption or the opening of a fissure in the earth is not unusual. The sky is a dark, starless void, but the landscape is weirdly lit by leaping flames all about. There are rivers of liquid fire, and at least two large lakes (all the bodies of liquid are interconnected), and there the flames blaze brightest.
This “water” is home to several wandering salamanders that were brought here long ago, to be bound in servitude to Belial. This didn’t work out – salamanders take orders from no creature when it does not suit them – and most of the salamanders were immediately slain by Belial’s servants and vassals. But a few escaped, yet bound to this plane by Belial’s magic, and survive by avoiding large groups of devils and snatching lone creatures who venture too near to one of the rivers of fire.
This plane is perhaps the most visited of the Nine Hells except for Avernus, since outsiders who work magic often come to the fabled “firefalls” of Phlegethos, where the rivers of fire tumble from volcanic ridges down into clefts. The fire in such a spot is mentioned by many alchemists and mages skilled in the creation of magic items.
Belial rules from the huge, hollowedout shell of a dead volcano. His stronghold- is known as Abriymoch, “The Mount of Leaping Flames.” The archdevil is attended by his consort Naome, his legate Chamo, and a few malebranche. Abriymoch consists of several tiers of chambers, opening into the central shaft like balconies, and connected to each other by stairs, shafts, and a great spiral path that winds about the central gulf or shaft of the dead volcano. Some chambers stretch through the sloping walls of the volcano and have windows or doors opening out onto the volcano’s outside flanks. The lips of the volcanic crater are crowned by Abriymoch’s basalt towers. A few erinyes under the command of the pit fiend Zaebos, Belial’s lieutenant, serve as messengers within Abriymoch and between it and vassal dukes or outlying patrols. These erinyes are often bullied and are quite miserable, and as usual have their eyes always on bettering their own positions in the hells. Chamo suspects that the erinyes have served as the spies of Asmodeus, Baalzebul, and perhaps other archdevils in the past.
Abriymoch is staffed by spined and bearded devils and has a kennel of hell hounds that can be used by Zapan’s forces to help defend and guard the palace. The vast, smoking plains surrounding the volcanic heart of Phlegethos are the domain of Belial’s vassals: Balan, who commands 40 companies of bearded devils; Bathym, who can muster 30 companies of barbed devils; and Gaziel, who leads 11 companies of bone devils. These vassals are usually dealt with and commanded by Zaebos, speaking for his master Belial, who, assisted by Chamo, is usually occupied with matters of diplomacy and intrigue within the Nine Hells.
Phlegethos is the most chaotic physical environment in the Nine Hells, affording intruders the most opportunities for concealment – but it is also one of the most active planes, being constantly traveled by its inhabitants. The archdevils of other planes often test underlings by sending them here, charged with accomplishing a task and remaining undetected by the patrols that roam Phlegethos, while their progress is observed by spies – of whom there are many among the barbed devils.
As with Minauros, the outlands of Phlegethos are wetter and lower than the interior, and these “reeking fens” (smelly, flooded, marshy lands) are populated by bearded devils, a few styx devils and abishai, hell hounds, lemures, and the mephits who flit (moves swiftly) about constantly everywhere on Phlegethos, serving as “eyes” for any and all who reward them.