Massacre is the twenty-fourth short story published in the Horus Heresy Series that was not originally part of an anthology novel. Massacre was first published for free on the Warhammer App over 5 days in July 2015, and later released as an e-book. Massacre is the prequel to the Warhammer 40,000 Night Lord Series by Black Library author Aaron Dembski-Bowden. It is now also included in the Eye of Terra anthology novel.