The Horus Heresy, sometimes referred to poetically as the "Age of Darkness", was a galaxy-spanning civil war that consumed the worlds of Mankind for 9 Terran years. Its outbreak marked the end of the Emperor of Mankind's Great Crusade to reunite the scattered colony worlds of Humanity in the Milky Way Galaxy under a single government and the beginning of the current Age of the Imperium. The Horus Heresy is in many ways the founding event of the Imperium of Man as it now exists.
