The Horus Heresy is a major event in the Warhammer 40,000 universe that occurred approximately 10,000 years before the current era.
It was a galaxy-wide civil war that lasted for more than a decade and was fought between the forces of the Emperor of Mankind and those of his former favorite son, Horus, who had been corrupted by the Chaos Gods.
Horus, who was the Warmaster of the Emperor’s armies, led a rebellion against the Imperium, and many other Space Marine Legions and Imperial Army units joined him in his cause. The war was brutal and resulted in the deaths of countless humans and Space Marines, as well as the destruction of many worlds.
The Horus Heresy was a turning point in the history of the Warhammer 40,000 universe. It shattered the unity of the Imperium, which had been created by the Emperor, and led to the current era of the Imperium’s decline, stagnation, and decay.
Some of the Space Marine Legions that fought in the Heresy were disbanded or destroyed, and others were divided into smaller Chapters. In addition, many of the primarchs, the superhuman leaders of the Space Marine Legions, were lost or killed during the Heresy, leaving the Imperium without their leadership and guidance.
The Heresy also led to the emergence of Chaos Space Marines, who were former loyalist Space Marines who had been corrupted by Chaos and had turned against the Imperium.