Mike Mignola's Hellboy is about a demon child summoned to Earth as its destructor. Instead, adopted by an expert in the occult, he grows up to be an occult investigator and general protector of the human race. In addition to spinning off several additional comic book series, Hellboy has had two major motion picture adaptations, directed by Guillermo del Toro, and an RPG adaptation published by Steve Jackson Games.
The Hellboy Sourcebook and Roleplaying Game is a stand-alone tabletop RPG that is compatible with GURPS Third Edition. For those interested in using it with the Fourth Edition rules, it could be used as a setting supplement for GURPS Monster Hunters. Alternately, characters can be updated to the Fourth Edition rules with the help of the GURPS Update conversion guide. The Fourth Edition equivalent of "ritual magic" is the Effect Shaping variant of Path/Book Magic in GURPS Thaumatology, but if you don't have that book, you can just use the original rules with little trouble. "Spell-based Magic" is the default magic system in the Basic Set. If you have any problems, you can get help on the Steve Jackson Games GURPS Forum.