Great Scouring

The Great Scouring was the Imperium of Man's great counter-offensive against the Traitor armies of the Warmaster Horus, following the defeat of the Horus Heresy after the Siege of the Imperial Palace on Terra in the early 31st Millennium.

History
Horus' campaign to strike towards Terra with his Traitor Legions and other Traitor forces during the Horus Heresy 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 Traitors were broken and in retreat towards the Eye of Terror. Before actually being confined within the life support mechanisms of the Golden Throne, the Emperor had pronounced judgment on the Traitors: 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 Isstvan V and Davin would be scoured clean of all life because of their corruption by Chaos. The Traitor Legions' associated troops from the Dark Mechanicum or the regiments and starships of the Imperial Army 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 years after the Heresy had ended with Horus' death on the bridge of his flagship 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, it had only 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 at the Drop Site Massacre on Istvaan V were slowly reestablished using what little gene-seed the survivors had managed to escape with, though the Legions were refashioned as 1,000-Space Marine Chapters rather than full Legions as Primarch Roboute Guilliman's Codex Astartes now required after the Second Founding.

With the Emperor confined to the Golden Throne and the Imperium 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 been ravaged by the fighting during the Heresy. The light of the Great Crusade 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 Imperial Truth favoured by the Emperor was jettisoned for the growing and once-illegal Imperial Cult, which 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 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 were a major factor in the final triumph of the Imperium over the Traitor Legions and the other Forces of Chaos, largely because they 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 against the Word Bearers Traitor Legion in the galactic east. Roboute Guilliman was the greatest military strategist alive after the Emperor's internment and he rallied the other 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. Roboute 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 Great Scouring was followed by the ten-millennia-long period known as the Age of the Imperium.