The Horus Heresy: Book One - Betrayal is the first book released as part of Forge World's Horus Heresy Series of campaign and army books that takes place during the Horus Heresy. The book deals with the events starting near the end of the Great Crusade and includes the Istvaan III Atrocity. It contains rules and special characters for the pre-Heresy Sons of Horus, World Eaters, Emperor's Children, and Death Guard Legions as well as a Space Marine Crusade Legion army list.