From the publisher's website:
The Black Libram of Nartarus is a unique new installment to the Castles & Crusades fantasy role playing game. It is a book of wicked secrets, intended not as a supplement for the player characters to use for their own devices, but instead as a tool for the Castle Keeper to unleash new levels of magic, monsters and mayhem upon her jaded and unsuspecting players. The spells and items contained within this book are all of a decidedly wicked and evil nature.
Use of such secrets by player characters would have an undoubtedly wicked and damning effect. In order to use this dark knowledge the characters would most undoubtedly have to be evil, or be forced by circumstances to have the need or desire to seek the knowledge contained upon these pages.
The Black Libram of Nartarus has its genesis in the Haunted Highlands campaign setting. The deities, items, rituals, and villains mentioned upon these pages are decidedly referenced from that setting. The magical lore found here is suitable for any mature themed campaign setting, played in by players mature enough to understand and accept its decidedly dark themes.
Beyond Castles & Crusades, the magic items and spells within this Libram may be of use in any fantasy role playing game with little work on the part of the game master.