The Horus Heresy
The civil war, ten thousand years before the current setting, in which half the primarchs turned on the Emperor and lost.
also known as: the heresy · heresy
The Emperor named Horus his Warmaster and went home to Terra. Horus fell to Chaos, took half his brothers with him, and the crusade to reunite humanity turned into a war that burned the young Imperium end to end. It finished at the Siege of Terra with Horus dead, the Emperor mortally wounded on a life-support throne, and the traitor legions fled into the Eye of Terror. Everything in the 41st millennium is downstream of it.