The Scouring was the long and bitter campaign that followed the Horus Heresy, in which the loyalist forces of the Imperium hunted the defeated traitors and drove them from the galaxy. With the Warmaster dead and the Siege of Terra broken, the surviving loyalist Legions turned from mere survival to vengeance, determined to purge the rebels from every world they had seized.
The fighting was vast and merciless, spanning decades as loyalists reclaimed contested territory system by system. The traitor Legions, robbed of their leader and lacking unity, were pushed relentlessly back toward the Eye of Terror, that great warp storm where the corrupting powers of Chaos held sway. There they made their final refuge, retreating into the immaterium to nurse their hatred and plot their return.
The Scouring did more than expel the traitors; it reshaped the Imperium that emerged from the war. In its course the surviving Space Marine Legions were broken up into smaller Chapters, and sweeping reforms were enacted to ensure no single warlord could ever again command such power. When the campaign at last wound down, the immediate threat was ended, but the traitors were not destroyed, only exiled, and from their dark sanctuary they would sally forth in black crusades for the ten thousand years to come.