From publisher blurb:
The traditional fantasy world of the world's first (and most popular) fantasy role-playing game is besieged by the supernatural. Undead, necromancers, evil magicians, demons, devils, and more. The Exorcist is dedicated to combating those evils.
In practice, the class falls somewhere between the Paladin and Ranger, although unlike either, does not cast spells. Indeed, the central class power of the Exorcist is that he is increasingly immune to hostile magic, gaining magic resistance as he gains levels.
Although primarily intended for the first advanced edition of the world's most popular fantasy role-playing game, it also contains tables for other early versions: original edition, basic and expert editions, and guidelines for the cyclopedic edition.