The Imperium
First Warnings
Black Crusade
The Gathering
The Imperium
(continued)
Armed with the Space Marine Legions, the Emperor's forces set about reconquering the galaxy. Worlds isolated for millenia were freed, or in some cases purged of Chaos or other alien infestation. The Primarchs were rediscovered, having each landed on a different planet and grown into the god-like men they were designed to be. Each was reunited with his Legion, taking command to further spread the Imperium.

Horus was the most powerful of Primarchs, and the Emperor's closest advisor. However, his embryonic capsule had been cracked and invaded by the corruption of Chaos. Once Horus had strengthened his Legion and gained control of a large portion of Imperial troops, he turned on the Emperor, attacking Earth in an attempt to destroy the Imperium for his Chaos Lords in one fell swoop. Other Legions fell to the temptations of Chaos, and soon a massive conflict erupted pitting Space Marine against Space Marine. In the end, the Chaos forces were repelled outside the realm of regular space. The Emperor killed Horus, but sustained fatal wounds himself. His nearly lifeless body was placed in a stasis field and encased in a throne of gold.

As a result of the Heresy, the Codex Astartes was created to elimitate the practices that allowed the conflict to happen. Control of the Imperium was transferred to the High Lords of Terra. The Space Marines continue to fight a neverending battle with the forces of Chaos, who invade human space when the unpredictable currents of the warp grant them access.