The Abyss is an evil plane that is the home of demons.
In the center of the Abyss is an incredibly deep pit which gives the plane its name. Around the pit, a vast array of caverns extend in all directions. To the north there's a frozen wasteland and ice caverns, and to the east a vast underground sea, but the majority of the plane is molten lava. The plane is also filled with a dark Soul Magic field which only demons and higher beings can channel safely.
The Abyss started out as its own plane millennia ago. A portal to the Abyss in the city of Stygia on Mezulbryst caused a connection between them, but the merging wasn't fully complete until the Planar Wars. In 4813, the Abyss was moved to Daresa by Keolah. It was then directly impacted by the Gold Star III under command of Ishane. When Shazmar returned, he sealed up the holes, the Abyss included.
Following the competition in 4816, Lakisval wished the Abyss back into existance as its own plane again, and Shazmar thereby restored it to its former glory.
The entrance to the Abyss was located on Mezulbryst, causing wars between demons and the local drow, up until Keolah moved it again in response to Riven Soulfire's prayer.[1]