The Abyss is the Chaotic Evil Lower Plane. It consists of 666 layers, and is primarily the home of Tanar'ri, or demons. TNO is spared the pleasure of visiting this plane, but not from visiting Baator. This plane is undoubtedly a whirlwind of chaos as tanar'ri struggle for dominance in this bleak realm. Demons follow the lead of the strongest and have little order in their society. Compare this with the ordered life in the aforementioned Baator.
Gygax, G. (1978). Advanced Dungeons & Dragons player's handbook. Lake Geneva, WI: TSR Games.