|This article is about the Imperial campaign, for the Tau campaign see Campaign of Scouring|
|Outcome||Imperial victory and mass purges, traitors flee to the Eye of Terror|
|Imperium||Forces of Chaos|
| Lord Commander Roboute Guilliman(WIA)
Primarch Lion El'Jonson(WIA)
Primarch Jaghatai Khan
Primarch Rogal Dorn
Primarch Corvus Corax
Primarch Leman Russ
| Ezekyle Abaddon|
Primarch Konrad Curze(KIA)
Loyal Imperial Army
| Sons of Horus|
Traitor Titan Legions
Traitor Imperial Army
|Extremely heavy||Extremely heavy|
The Great Scouring was the Imperium's counter-offensive against the rebel armies of Horus, following the defeat of the Horus Heresy in c.M31. Horus's campaign to strike towards Terra to achieve a quick and decisive victory had ultimately been defeated. The rebels were broken and in retreat, and Imperial forces now had the job of reacquiring all the territory they had captured. The Great Scouring saw the first mass galaxy-wide purge carried out by the Imperium as it sought to erase the taint of Horus' treachery from its realm.
Before actually being confined within the Golden Throne, the Emperor had pronounced judgment on the rebels: they were to be driven into the hellish region of 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 and the Traitors' associated troops were to be destroyed or driven into the Eye. It would be as if the Traitor Legions never existed. Meanwhile since the death of Horus, the Traitor Legions had become splintered and dispersed.
Fighting continued for another seven years before the rebels were wholly destroyed or exiled. The Imperium fought not only the traitors but also Xenos that had taken advantage of the turmoil caused by the Heresy to plunder human worlds. Systems once thought unassailable to the Xenos threat were put under siege. During the campaign, loyalist forces realized the sheer scale of Chaos influence in the Imperium as world after world was discovered to have traitor presence. Many corrupted systems were cleansed and placed under the watch of the Inquisition. Horus's death had not ended the fighting, it had renewed the loyalists against the rebels. Many worlds during the Heresy had refused to commit their forces to either side, or seceded entirely. Such indecision was punished by loyalist and rebel alike. These forces were bled white attacking rebel strongholds. Meanwhile, the Legions destroyed at the Drop Site Massacre were slowly reestablished using what little gene-seed the survivors had managed to escape with.
With the Emperor confined to the Golden Throne and the Imperium leaderless but loosely organized under Roboute Guilliman, the forces of Chaos and all of the enemies of the Imperium swept down on the defenceless worlds which had been ravaged by the fighting during the Heresy. The light of the Great Crusade was gone to be replaced by a dark time in which death was far more prevalent and hope seemed far away. The forces of the Imperium were reorganised and redeployed and gradually the followers of Chaos were forced to flee west to the Eye of Terror. Eventually the Imperium succeeded in stabilising itself, but not without heavy losses of men and machinery.
The Ultramarines were a major factor in the success of the Imperium owing to their not taking part in the Siege of Terra due to fighting in the galactic east. Roboute Guilliman was the greatest military strategist alive and rallied the other Space Marine Legions to act as a bulwark against all enemies of the Imperium. The days following the Heresy were to be some of the darkest seen since the Emperor rose to power. Guilliman was said to have been everywhere during this period, rallying forces 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 and Guilliman's tactics.
During the Scouring, great political changes were introduced into the Imperium, primarily overseen by Guilliman. The most significant of these was breaking apart the Space Marine Legions into Chapters as well as dividing the Imperial Army into the Imperial Guard and Imperial Navy. Both of these measures were intended to curb the power that any single man could wield in the Imperium, limiting the damage if another rebellion broke out.
Notable events during the Great Scouring
- The Lion El'Jonson goes missing following the Destruction of Caliban and the Dark Angels begin their hunt for The Fallen.
- In a battle known as the Iron Cage the Imperial Fists take heavy losses assaulting Perturabo's trap on Sebastus IV[9a]
- Planets associated with the Traitor forces are subjected to Exterminatus. These include Colchis[7a], Olympia[9a], Cthonia[7b], and Chemos[9b].
- Alpharius or Omegon is believed to be killed by Roboute Guilliman on Eskrador, but the twin Primarch's fate remains unclear.
- Jaghatai Khan is thought lost in the Webway during battles against Dark Eldar raiders in the Yasan Sector.
- While leading a genocidal rampage across the Eastern Fringe, Konrad Curze is killed by an agent of the Officio Assassinorum.
- Roboute Guilliman is mortally wounded by Fulgrim during the Battle of Thessala and put into stasis.
- An unknown force destroys much of the Nostramo Sector
- Rogal Dorn is thought killed in a Black Crusade
- The Battle of Exyrion between the Dark Angels and an alliance of Iron Warriors and Fallen
- 1:Warhammer 40,000 5th Edition Rulebook, p. 124
- 2:Realm of Chaos: Slaves to Darkness (1988), p. 243
- 3:Codex: Space Marines (4th Edition)
- 4:Warhammer 40,000 6th Edition Rulebook pg.161
- 5: [[Warhammer 40,000 7th Edition Rulebook] - The Age of the Imperium
- 6: Nightbringer (Novel) - The Ultramarines Omnibus (Omnibus) Ch. Two, p. 49
- 7: Index Astartes IV - The Enemy Within - The Alpha Legion Space Marine Legion
- 8: Index Astartes II - Bringers of Darkness - The Night Lords Space Marine Legion
- 9: Index Astartes I - The Unforgiven
- 10: Codex: Space Marines (8th Edition) pg.34