Avernus

Avernus serves as the front line of the great clash between the evil forces of chaos and law known as the Blood War. Demons from the Abyss use the River Styx to enter Avernus, where they hurl themselves against infernal legions of devils.  Avernus was not always the blasted, war-torn battlefield it is today. Long ago, it was a plane of lush gardens and bucolic beauty created by Asmodeus to tempt mortals. The intrusion of the River Styx followed by endless waves of slavering demons destroyed this paradise, leaving layer upon layer of bones, ruins, and shattered war machines. Pieces of cities stolen from other planes, tiny remnants of Avernus's lost beauty, evidence of destroyed celestial armies, and tombs of ancient travelers all dot the Avernian Wasteland.
Avernus is not a place expressly for the punishment of evil souls; instead, it represents a "next level" for evil souls after the end of their wicked lives as mortals. The Nine Hells allows souls to exist in a concentrated form, where they can work through their spiritual bondage to express their dark desires such as cruelty, covetousness, the need to control, the craving for power, and uncontrolled greed. Hell offers an eternity of fulfillment from the most insignificant desires to the seven deadly sins. Every shred of evil is used in the Nine Hells, and each layer specializes in some way to accommodate abd exploit the vices and weaknesses of mortals. The devils of Avernus seek to exploit pride and wrath, promising the aggrieved, enraged, and egotistical the power to fulfill their darkest obsessions. Such fulfillment, however comes at a price.
Anyone entering Avernus finds a battlefield unlike anywhere else in existence. Evidence of past carnage, such as the ruins of enormous war machines and fields of bleached bones, stretches across the horizon. Through it all, the River Styx winds its way across the plane as knots of demons and devils skirmish along its fetid banks.
Most devils in Avernus serve Zariel, though other Lords of the Nine send forces here to help repel demonic invaders. Devils who are not part of Zariel's legions serve as emissaries from the other layers or as spies for archdevils seeking signs of weakness that they can exploit to further their dreams of power.
(Baldur's Gate: Descent into Avernus, 2019, pg 75)

Type

Dimensional plane