|Attacker||Imperium of Man|
|Attacking Commander||Roboute Guilliman, Rogal Dorn, Leman Russ, Jaghatai Khan, Vulkan, Corax, many other Imperial heroes|
|Defending Commander||Abaddon the Despoiler, Mortarion, Perturabo, Fulgrim, Magnus the Red, Angron, Konrad Curze, Lorgar, Alpharius Omegon|
|Theatre||Multiple Sectors across the Milky Way Galaxy|
|Strength (attackers)||9 Loyalist Space Marine Legions, Imperial Army, Collegia Titanica, Adeptus Mechanicus military forces, Sisters of Silence, Officio Assassinorum|
|Strength (defenders)||9 Traitor Legions, Traitor Imperial Army, Traitor Titan Legions, Dark Mechanicus military forces, Daemons, Mutants, Chaos Cults|
|Casualties (attackers)||2.3 trillion dead from Imperial purges|
|Outcome||Forces of Chaos driven from Imperial space into the Eye of Terror; unity of Traitor Legions shattered|
The Great Scouring, or simply The Scouring, was the Imperium of Man's great counter-offensive against the Traitor Legions. It began immediately after the end of the Horus Heresy following the death of the Warmaster Horus and the failure of his Siege of the Imperial Palace during the Battle of Terra in the early decades of the 31st Millennium. It succeeded in driving the Forces of Chaos from Imperial space into a permanent refuge in the Warp rift known as the Eye of Terror.
The bitter fighting of the Horus Heresy was only the beginning of a new age for Mankind founded in blood, battle and the living enshrinement of the Emperor as a God. After the death of their leader, Horus, the Traitors splintered and dispersed, but many hard battles remained. Horus' campaign to strike towards Terra with his Traitor Legions and other Traitor forces to achieve a quick and decisive victory that would overthrow the rule of the Emperor of Mankind had ultimately been defeated during the Battle of Terra. The fledgling Imperium was wrested back from the brink of destruction by those who remained staunchly loyal to the Emperor's vision for humanity. After the death of Horus, those Traitors who had not been slain outright during the Siege of the Imperial Palace fled before the vengeful wrath of the Loyalist forces. Many made good their escape into unexplored space or disappeared into the Eye of Terror or other, lesser-known Warp rifts such as the Maelstrom.
Before actually being confined for all time within the life support mechanisms of the Golden Throne, the Emperor had pronounced judgment on the Traitors: he declared them Excommunicate Traitoris, and determined that they were to be driven into the hellish region of the Warp rift called the Eye of Terror, which would hold them for all eternity. All records and memory of the Traitor Legions were to be expunged from the Imperial archives. Worlds such as Istvaan V and Davin would be scoured clean of all life because of their corruption by Chaos. The Traitor Legions' associated troops from the Dark Mechanicus, the Titan Legions or the regiments and starships of the Imperial Army and Imperial Navy that had turned to Chaos were to be destroyed or driven into the Eye. It would be as if the Traitor Legions had never existed to sully the Imperium with their betrayal.
Fighting continued for another seven standard years after the Heresy had ended with Horus' death on the bridge of his great flagship, the Vengeful Spirit, before the Traitor forces were wholly destroyed or exiled into the Eye of Terror. Many Chaos-corrupted star systems were cleansed and placed under the watch of the newborn Inquisition. Horus' death had not ended the fighting, but it had renewed the resolve of the Loyalists to destroy the Traitors. Many Imperial worlds during the Heresy had refused to commit their forces to either side, or seceded entirely from the Imperium to regain their independence. Such indecision was punished by Loyalist and Traitor forces alike. These forces were often bled white attacking the rebel strongholds of worlds that only wanted to be free of the Imperium entirely, whether it swore allegiance to the Emperor or to the Dark Gods. The Loyalist Space Marine Legions destroyed during the Drop Site Massacre on Istvaan V -- the Raven Guard, Iron Hands and Salamanders -- were slowly reestablished using what little gene-seed the survivors had managed to escape with, though the Legions were refashioned as Space Marine Chapters comprised of only 1,000 Astartes rather than full Legions as Primarch Roboute Guilliman's Codex Astartes required after the Second Founding and the beginning of what was known as the Reformation of the Imperium.
With the Emperor confined to the Golden Throne and the Imperium left effectively leaderless, the Forces of Chaos and all of the enemies of the Imperium like the Orks and Eldar pirates swept down on the defenceless worlds which had already been ravaged by the terrible internecine fighting during the Heresy. The light and hope of the Great Crusade era was gone forever, to be replaced by the dark time known as the Age of the Imperium in which death was far more prevalent, war was the only constant in the galaxy and hope seemed far away. Without the Emperor to lead humanity directly, the sheer threats that the galaxy presented to human life led the High Lords of Terra to create an Imperial administration that was increasingly authoritarian, highly bureaucratic, ponderous and often uncaring towards individual human lives as long as its authority was maintained. Religion also became an important tool of social control as the original rationalist and atheistic Imperial Truth favoured by the Emperor was jettisoned for the growing and once-illegal Imperial Cult. The growth of this church cemented loyalty to the Imperium amongst the teeming masses of humanity at the same time that religious faith provided a modicum of comfort against the often harsh realities of the galaxy. The military forces of the Imperium were reorganised and redeployed and gradually the remaining followers of Chaos were forced to flee into the galactic west, to the massive Warp rift of the Eye of Terror. Eventually the Imperium succeeded in stabilising itself, but not without heavy losses of men and materiel.
The Ultramarines Legion proved to be a major factor in the final triumph of the Imperium over the Traitor Legions and the other Forces of Chaos. This was largely because their numbers had not been whittled down through heavy attrition during the Battle of Terra due to the fighting that had consumed them in the Calth System in the galactic east against the Word Bearers Traitor Legion. Roboute Guilliman, the Primarch of the Ultramarines, was the greatest human military strategist alive after the Emperor's internment and he rallied the other Loyalist Space Marine Legions to act as a bulwark against all enemies of the Imperium. The days following the Horus Heresy were to be some of the darkest seen since the Emperor had risen to power on Terra during the Unification Wars at the end of the Age of Strife. Guilliman was said to have been everywhere during this period, rallying the Imperial defenders and reinforcing them with his Ultramarines before moving on to the next battle zone. Never once did the Space Marines take a step back, so strong was their faith in the Imperium, the Emperor and Guilliman's tactics.
The Emperor's dream of a new age of enlightenment, a time when Mankind was to be free from superstition and ignorance, has turnedout very differently for the galactic Imperium he founded. The Great Scouring was followed by the ten millennia long era known as the Age of the Imperium. It is an age of tyrants and of unreason, an era of stagnation and intolerance. His people have regressed into religious obfuscation while the Emperor sits immobile, his thoughts unknown; the immortal being whose will extends over a million worlds is unable to lift even a shrivelled finger. Though the Imperium succeeded in preventing its death at the hands of the Forces of Chaos during the Heresy, the cost of that conflict was the better future for humanity that the Emperor had hoped to create.
- Codex: Space Marines (4th Edition)
- Codex: Ultramarines (2nd Edition)
- Realm of Chaos: Slaves to Darkness (1988), p. 243
- Warhammer 40,000 Rulebook (6th Edition), pp. 161-162
- Warhammer 40,000 Rulebook (5th Edition), p. 124