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.
The civil war began following the corruption by the Ruinous Powers of Chaos of the Imperial Warmaster Horus Lupercal, the primarch of the Sons of Horus Legion and the most beloved genetic son of the Emperor. The Chaos Gods fed Horus’ innate ambition and sense of betrayal by the Emperor until he turned upon his father and sought to claim the Imperium of Man for his own.
Horus convinced half of the other primarchs and Space Marine Legions, as well as large swathes of the Imperial Army and the forces of the ancient Mechanicum, to turn Traitor alongside him and unleash a cataclysmic attempt to conquer the galaxy in his name.
