Avernus is a never-ending ruin of destruction and pain.
Avernus is the Realm of Dark, the home of the sadistic Demons and the dominion of the god Mephisto. A realm of chaos and shadows, it is punctuated by the screams of the tortured souls trapped in the palaces of bone and obsidian that puncture the unholy ground.
Avernus is the place where the souls of truley vile mortals and those who have been corrupted by Dark magic and Mephisto are tortured and warped into demons, sometimes at their own request. The demons work as envoys of Mephisto in the Great Game, corrupting human souls to bring them to hell once they have died and become demonic themselves over time, living to serve Mephisto. Avernus can be a rewarding place for those who chose the darkness and utterly gave themselves to it, for those with not a scrap of virtue left can indulge in their darkest desires. For the souls who are not completely corrupted, however, every ounce of good is torn from them in agony.
Avernus is the closest Realm to the Abyss, and ruptures frequently tear between them, letting the Others pour in vengefully. In these situations the battles are bloody but efficient, demon screamers tearing their way through the invaders.