The Horus Heresy

The Horus Heresy was the great betrayal, the wound at the heart of the Imperium from which it has never healed. Horus Lupercal, the Warmaster and best-beloved son of the Emperor, was seduced by the Ruinous Powers of Chaos and turned his ambition against the father who had raised him above all others.

Half the Primarchs and their Legions followed Horus into damnation, becoming the first traitors and forebears of the Chaos Space Marines. The World Eaters, Death Guard, Emperor's Children, and Thousand Sons among them fell to the four dark gods, unleashing atrocity upon their loyalist brothers in a war that set brother against brother across the stars.

The treachery was baptised in fire upon Istvaan, where loyal warriors were massacred by those they had called kin. From there the rebellion swept toward Terra itself, worlds burning in its wake as the loyal Space Marines fought a desperate retreat toward the cradle of mankind, buying time in blood.

The war reached its terrible climax in the Siege of Terra, where Horus assailed the Emperor's own palace. In the final confrontation aboard the Warmaster's battle-barge, the Emperor slew His fallen son but was maimed unto near-death, His broken body interred forever upon the Golden Throne. The dream of the Great Crusade died there, and the grim, sunless Imperium of ten thousand years of war was born from its ashes.