Great Scouring
From Lexicanum
The Great Scouring directly followed the Horus Heresy. With the Emperor confined to the Golden Throne and the Imperium leaderless, 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 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.1
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.1
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.1
The Great Scouring was followed by the period known as the Age of the Imperium.1
