The Scouring was the long and bitter reckoning that followed the Siege of Terra, the sprawling campaign in which the victorious loyalists hunted down and destroyed the shattered remnants of Horus's great rebellion. With the Warmaster himself slain and the Emperor entombed forever upon the Golden Throne, the war did not so much end as change its bloody shape, becoming a galaxy-wide pursuit of vengeance and retribution.
The traitor Legions, broken at Terra but very far from destroyed, fought a long and savage fighting retreat back toward their scattered strongholds. The vengeful loyalist Space Marines, reinforced and burning with grief, drove them from world after world in grim campaigns of retribution, reclaiming the vast territory the rebellion had seized and putting the servants of the Dark Gods to the sword wherever they could be run to ground.
World by world and system by system, the traitors were steadily pushed toward the great and howling warp storm that would in time become their eternal prison. At the last the surviving rebel Legions were driven within the Eye of Terror, that hellish rent in reality where they would fester, scheme and rebuild across the ten thousand years to come, launching their dreaded Black Crusades against the Imperium ever after. Not every traitor could be run to ground, and dark seeds were left scattered across a hundred worlds to trouble the ages still to come.
The Scouring cleansed the galaxy of open rebellion, but it also hardened the grieving Imperium into something colder, crueller and far more fearful than the enlightened realm the Emperor had once envisioned. Out of its ashes came the great and paranoid reforms that would shape the whole age to follow, a bleak legacy chronicled in the wider history of the Horus Heresy.