In the fictional Warhammer 40,000 universe a Black Crusade is when the forces of Chaos manage to unite under a particularly strong leader and go forth en masse to wage war against the Imperium of Man. There have been thirteen Black Crusades to date, all started by Abaddon the Despoiler, Warmaster of the Black Legion and the (rumoured) clone-son of Horus.
Prior to the 13th Black Crusade, each of Abaddon's 12 assaults had failed. The 13th Black Crusade was the background to the world-wide Warhammer 40,000 Eye of Terror campaign, which resulted in a minor victory for the forces of Chaos. Now having gained a foothold on the Cadian system, he is able to strike out and attack from real space, as opposed to traversing the Immaterium.