The Horus Heresy is a significant event in the lore of the Warhammer 40,000 universe, occurring in the 31st millennium. It involves a galaxy-wide civil war initiated by Horus, the favored son of the Emperor of Mankind. Horus, corrupted by the forces of Chaos, leads several of the Emperor's Space Marine legions against the Imperium, resulting in a brutal conflict that reshapes the galaxy. This heresy marks the beginning of the long-standing division between loyalist and traitor factions within the Imperium. The aftermath leads to the Emperor's internment in the Golden Throne and the establishment of the Imperial Cult, fundamentally altering the course of human history in the Warhammer 40,000 setting.