The 1st Black Crusade occurred in the late 31st Millennium, five standard centuries after the end of the Horus Heresy. Utilising the region of calm space around the Eye of Terror known as the Cadian Gate, Abaddon led the combined forces of the Chaos Space Marine Traitor Legions against scores of Imperial worlds, and was responsible for millions of deaths in the first week alone. Multiple worlds were lost before the Imperium could launch an effective counterassault, and although the Black Crusade was ultimately defeated, the Imperium was forced to heavily fortify the surrounding Cadian Sector, transforming Cadia itself into a Fortress World.