The comprehensive sourcebook of demons in the D&D world.

While the text is heavily Greyhawk-centric (mentioning Iuz, Greyhawk's version of Lolth etc), core information (such as demons and two new subtypes, Loumara and Obyrith) can be easily ported to other game worlds as well.

The book is presented as a result of "in-character" point of view study where it doesn't discuss game statistics or game-related advice.

Included are: feats (new type:Abyssal Heritor feats), prestige classes, new versions of some old good demons, demon lords, layers of the Abyss, magical items, spells etc.