To start using Abyssal in game, type -ab console command for Mith's Virtual Console.
Abyssal is the common tongue of The Abyss, of Demons. Any priest or (un)holy warrior who deals regularly with creatures of The Abyss will learn this tongue as a means of communicating with the lower denizens. Of course also humanoids with Demon blood in them know this language. In short, Abyssal is a common tongue of planar, chaotic evil.
On Arelith this language is known by: