The Black Legion series is a planned trilogy of novels, written by Aaron Dembski-Bowden focusing on Ezekyle Abaddon, former First Captain of the Sons of Horus, in the immediate aftermath of the Horus Heresy.
The series is one of several published concurrently by Black Library, taking place in the aftermath of the Heresy and featuring several prominent Chaos Space Marines as the "heroes" of their stories, such as Ahzek Ahriman and Khârn the Betrayer.
