The Horus Heresy was the catastrophic civil war that shattered the young Imperium in the closing years of the 31st Millennium, pitting the Emperor's loyal servants against half of their own kind. Its name derives from Horus Lupercal, the Warmaster and most favoured of the primarch sons, whose corruption by the Ruinous Powers turned brother against brother.
Seduced by Chaos and nursing wounds of pride and ambition, Horus drew nine of the twenty Space Marine Legions into rebellion. What began as hidden treachery erupted into open war, its opening atrocity the massacre of loyalists by former comrades. From there the fighting spread across countless worlds, each Legion's turning a fresh wound in the realm the Emperor had spent centuries building.
The war climaxed in the desperate defence of humanity's cradle, as the traitor host besieged the Throneworld itself. In a final duel aboard Horus's flagship, the Emperor destroyed his favoured son but was maimed beyond recovery, condemned to an eternal half-life upon the Golden Throne. The surviving traitor Legions fled into the Eye of Terror, and the Imperium that endured became a grief-stricken, dogmatic shadow of the enlightened dominion once envisioned. For a fuller account, see the Horus Heresy explained.