The Horus Heresy is a major storyline within the fictional universe of Warhammer 40,000, a tabletop wargame created by Games Workshop. It also encompasses various novels, audio dramas, and other media. The Horus Heresy is a pivotal event in the lore of Warhammer 40,000 and serves as a foundation for much of the game's setting.
In the Warhammer 40,000 universe, the Horus Heresy is a galaxy-spanning civil war that occurred approximately 10,000 years before the current timeline of the game. It was a catastrophic conflict that nearly resulted in the total destruction of humanity and the downfall of the Imperium of Man, a vast and authoritarian interstellar empire.
The Horus Heresy began when the Warmaster Horus Lupercal, one of the Emperor of Mankind's most trusted and powerful Primarchs, rebelled against the Emperor's rule. This rebellion led to a schism within the ranks of the Space Marine Legions, the elite super-soldier forces of the Imperium, as many of them followed Horus in his betrayal.
The conflict involved epic battles, political intrigue, and the corruption of powerful beings by dark forces, such as the Chaos Gods. It also saw the emergence of various traitor Space Marine Legions, who turned against their former allies and loyalist Legions.
Ultimately, the Horus Heresy ended in a climactic battle known as the Siege of Terra, where Horus led a massive assault on the Emperor's palace on Earth (Terra). The Emperor and his loyalist forces managed to defeat Horus, but at a great cost. The Emperor was mortally wounded and placed on the Golden Throne, a life-sustaining device that keeps him alive but in a comatose state, guiding the Imperium from the shadows as a near-dead god-like figure.
The Horus Heresy left the Imperium shattered and scarred, and it marked the beginning of a grim and dystopian era in the Warhammer 40,000 universe. The Traitor Legions and the forces of Chaos continue to threaten humanity, and the galaxy is plagued by constant warfare, xenos threats, and the relentless march of time. The aftermath of the Horus Heresy is a central theme in the lore of Warhammer 40,000, and it shapes the setting's dark and grim tone.