Roboute Guilliman continued to serve with the Ultramarines Chapter, leading them for a hundred years after the Second Founding. It was said that during those years, Gulliman led several incursions, alongside with his brother Primarchs, against the remaining Chaos Space Marines. During one incursion, Gulliman faced Alpharius of the Alpha Legion and defeated him, but could not liberate the planet as the Alpha Legion were adept to fighting independently and eventually the Ultramarines were soundly beaten, despite the death of Alpharius. He was finally defeated by one of the traitor Primarchs, Fulgrim of the Emperor's Children, who had become a Daemon Prince of Slaanesh.
Fatally poisoned by his onetime brother, Roboute was transported back to Macragge in a stasis field, and has remained entombed in the field for ten thousand years. Although physically impossible in a stasis field, it is believed that his wounds are healing, and one day he will awaken again.