In the fictional Warhammer 40,000 universe a Black Crusade is when the forces of Chaos manage to unite under the leadership of Abaddon the Despoiler, Warmaster of the Black Legion, and launch a massive attack from within the Eye of Terror against the Imperium.
The first Black Crusade occured in the middle of the 31st millennium. Utilising the region of calm known as the Cadian Gate, Abaddon led the combined forces of the Traitor Legions against scores of worlds, and was responsible for millions of deaths in the first week alone. Multiple worlds were lost before the Imperium could fight back effectively, and although the Black Crusade was defeated, the Imperium was forced to heavily fortify the surrounding Cadian sector.
Over the course of the intervening ten thousand years, Abaddon has launched a further eleven Black Crusades, each one meeting in failure. Millions of other attacks have been made on the worlds of the Cadian Gate by Deamon Princes and lesser Chaos warlords, but the title of Black Crusade is reserved for the times when Abaddon seems to be able to unite all of Chaos in one goal.
The 41st Millennium saw two Black Crusades. The Twelfth Black Crusade, also known as the Gothic War, was Abaddon's attempt to capture the ancient Eldar artifacts known as the Talismans of Vaul, but known to the Imperium as the six Blackstone Fortresses. The Eldar allied themselves with the Imperium to defeat the Chaos forces, and were successful, although Abaddon escaped with at least one Blackstone Fortress, and possibly as many as three.
The Thirteenth Black Crusade began during the 999th year of the 41st millennium, and resulted in the largest mobilisation of both Imperial and Chaotic forces seen since the Horus Heresy. The Thirteenth 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 in the Cadian sector, Abaddon is able to strike out and attack from real space.