Horus Heresy
Origins of the Horus Heresy
The Horus Heresy, also known as the Great Betrayal, is a significant event in the Warhammer 40,000 universe. It was a galaxy-spanning civil war that consumed the worlds of the Imperium of Man 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 Great Crusade
The Great Crusade was a period of expansion and reunification for humanity. The Emperor of Mankind, a powerful psychic being, had united the warring factions of Terra, now known as Earth, and set out to reclaim humanity's lost colonies. He created the Primarchs, twenty superhuman beings, to lead his Legiones Astartes, commonly known as Space Marines, in this task. However, the forces of Chaos scattered the infant Primarchs across the galaxy. The Emperor found and reunited with them over the course of the Great Crusade.
The Primarchs and their Legions
Each of the Primarchs was a unique being, imbued with incredible physical, psychic, and intellectual abilities. They were designed to be superior to any human in every conceivable way, save for the Emperor himself. The Primarchs were the Emperor's finest creations, his sons, each imbued with a portion of his own essence. Each Primarch was given command of a Legion of the Adeptus Astartes, created from their own genetic material.
The Warmaster
The Emperor appointed Horus, the Primarch of the Luna Wolves (later renamed the Sons of Horus), as Warmaster, giving him command over the entire Great Crusade. Horus was the most accomplished of all the Primarchs and was the Emperor's favored son. However, this appointment sowed seeds of discontent among some of the other Primarchs, who were envious of Horus' position.
The Fall of Horus
Horus was mortally wounded during a battle on the moon of Davin. He was healed by the local tribes, who were under the influence of Chaos. During his recovery, Horus was shown a vision of the future where the Emperor was worshipped as a god and the Primarchs were forgotten. This vision, combined with his resentment towards the Emperor, made Horus susceptible to the temptations of Chaos.
The Heresy Begins
Horus, now corrupted by Chaos, turned against the Emperor. He managed to sway half of the Primarchs and their Legions to his cause, either through manipulation, promises of power, or revealing the truth about their creation. This marked the beginning of the Horus Heresy.
The Battle of Isstvan III
The first major conflict of the Horus Heresy was the Battle of Isstvan III. Horus purged his forces of those still loyal to the Emperor by sending them to the surface of Isstvan III and then virus-bombing the planet. However, some survived and managed to warn the Imperium about Horus' betrayal.
The Siege of Terra
The culmination of the Horus Heresy was the Siege of Terra. Horus' forces besieged the Imperial Palace, and Horus himself faced the Emperor in single combat. The Emperor, reluctant to kill his favored son, was severely wounded before finally slaying Horus.
Aftermath
The Horus Heresy had devastating consequences for the Imperium. The Emperor was interred within the Golden Throne, a life-sustaining apparatus, and has remained there ever since. The Traitor Legions were driven into the Eye of Terror, a region of space where the Warp and real space overlap. The Imperium was left in a state of constant war, beset on all sides by aliens, heretics, and the forces of Chaos.