The Horus Heresy: Book Two - Massacre is the second 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 of the Drop Site Massacre of Istvaan V. The book contains rules and special characters for the pre-Heresy Iron Hands, Salamanders, Night Lords, and Word Bearers Legions as well as an updated Space Marine Crusade Legion army list.