The Horus Heresy 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 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.

Over 9 Terran years the war raged, 7 of them consumed by Horus’ drive on the Throneworld, until coming to a final cataclysm on Terra itself where Horus was slain and the Emperor mortally wounded, His dreams of creating a brighter future for Mankind forever broken.

The conflict was fought across the Milky Way Galaxy early in the first century of the 31st Millennium and resulted in more than 2.3 trillion dead, 4.6 trillion if one includes the planetary populations purged by the Imperium after the Heresy due to the taint of Chaos corruption.

The Heresy concluded with the death of the traitorous Warmaster Horus, the internment of the Emperor in the Golden Throne and the exile into the Eye of Terror of the Heretic Astartes Traitor Legions.