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, once a loyal commander, turns against the Emperor, leading several Space Marine Legions to rebel. This conflict results in massive battles and the eventual fracturing of the Imperium of Man. The Heresy is marked by the betrayal of loyalist forces and the rise of the Chaos Gods, who manipulate events to weaken humanity. The aftermath of the Heresy shapes the future of the galaxy, leading to the establishment of the Imperial Cult and the ongoing struggle between the forces of Chaos and the Imperium.