For 10,000 years, Imperials cursed the name of Abaddon the Despoiler, a warrior of Chaos so reviled that Imperials despise him nearly as much as Horus himself. Of the 13 Black Crusades, nearly all dealt a major blow against the Imperium of Man (even if he hasn’t yet laid siege to Terra in his role as Warmaster) and even the demon princes off the warp understand that Abaddon the Despoiler is favored by the dark gods.

This theory has been confirmed in canon in the novel Soul Hunter by Aaron Dembski-Bowden. The prologue of the novel describes the transition of the Night Lord, Talos, into a space marine. One quick scene describes his early stages of genetic alteration as he stood naked in a room with many other initiates.