Horus

Horus was the first of the Primarchs to be recovered during the Great Crusade and for thirty years he, Serenity and the Emperor formed a trinity at the heart of the Imperium's leadership. His Legion were the Luna Wolves.

Several continuities have him falling to Chaos out of misguided fears for Serenity's safety and being redeemed by her.

In Heresy & Rebirth Horus is appointed Warmaster and his legion are renamed as the Sons of Horus. After Erebus of the Word Bearers lures him into service of Chaos, he turns the Space Wolves against the Thousand Sons and the Ultramarines against the Iron Warriors before striking at Terra. Having failed to take the Imperial Palace by assault, he lowers the shields of his battle barge to invite attack by the Emperor. The ship is boarded by Angron, Rogal Dorn, the Emperor and Serenity. In succession, Horus kills Angron, mortally wounds the Emperor and weakens Serenity to the point that Chaos can banish her from the universe. However, Serenity's last act before this is to cleanse Horus and many of his followers of their corruption. Consumed by guilt, Horus founds the Black Legion as a place for penitent traitors to serve the Imperium and ultimately dies amongst their ranks. He is reincarnated in the 41st Millenium as Teleute.