Avernus is the world/realm surrounding the the gaping opening of the endless Abyss. It is considered to be at the "bottom" of Limbo, and is a semi-stable area corrupted by demons and fiends. Some call it Hell (although there are a multitude of hells across the cosmos) It is a scorched and barren wasteland featuring rocky terrain, magma rivers, twisted vegetation, and a sickly red sky.
The ground is heavily pocked with thousands of dark, circular holes and pits that act as portals to other abyssal layers. Some go both ways, while others are one-way drops—making jumping into an unlabeled pit a dangerous gamble. Massive iron citadels and guard towers dot the terrain, forged out of molten iron lakes by lesser demon lords who try to secure portal access.
There is no ruler of Avernus, although the demon known as Pazuzu makes a strong claim for domain over the sky and the numerous floating islands and rocky crags that float across the it. The demon Amon controls a portal to Acheron through which he launches his attempts to conquer that realm. (Past Acheron lie the realms of Celestia , the Seven Heavens)
The Demon Warlock Kings also maintain a cluster of individual domains out in the wilds of Avernus, and it is said that Tiamat maintains a vast realm named Tehom Dilmon or is imprisoned here in the Pits of Azharul as a watchdog against the Abyss.
River Styx: The bruised purple or blood-red waters of the River Styx wind through Avernus, branching into tributaries and plunging down into the pits, erasing the memories of anyone who drinks from them. Thier source is reputed to be a vast underground realm called Nessus.
