In
AD&D, the
Outer Plane of
Chaotic Evil. Traditionally subdivided into
666 sub-planes, although I've also heard it said that the number of planes of the Abyss is
infinite. In a polytheistic fiction, 666 seemed a little out of place anyway. Inhabited and ruled by
demons (called
tanar'ri in
AD&D 2E).
AD&D: the Outer Planes