This 120-page hardback book contains:
Background and lore:A deep dive on the Talons of the Emperor, including the Legio Custodes, Anathema Psykana, and Divisio Assassinorum.
Rules and unit profiles:A guide to army building, plus rules for units, weapons, and Wargear available to the Legio Custodes in Warhammer: The Horus Heresy.
Detachments:Rules for five Detachments for the Legio Custodes: Moritoi Convocation, Hykanatoi Convocation, Tharanatoi Convocation, Ephoroi Convocation, and Kataphractoi Convocation.
Army Lists:Guidelines for building armies of Legio Custodes, Anathema Psykana, and Agents of the Divisio Assassinorum, as well as Advanced Reactions and Gambits.

You will need a copy ofWarhammer: The Horus Heresy Age of Darkness Rulebook, available separately, to use the contents of this book.