Roboute Guilliman, known as The Avenging Son, is the Primarch of the Ultramarines Chapter of Space Marines. Not only did he lead the largest Space Marine Legion during the Great Crusade, Guilliman was renowned for his efforts to preserve the Imperium in the aftermath of the devastating Horus Heresy. He ultimately became Lord Commander of the Imperium. However, he was mortally wounded during the Battle of Thessala and put into stasis thereafter by his surviving Chapter brethren.

Centuries later, after Abaddon the Despoiler had launched his Thirteenth Black Crusade and the destroyed of the planet Cadia, the Tech-Priest Belisarius Cawl and Saint Celestine led a group of Imperial survivors into the Eldar Webway. The group ultimately arrived at Macragge, the Ultramarines’ homeworld. Here Cawl revealed that millennia ago he had, in fact, been ordered by Guilliman to complete two tasks in the event of the Primarch’s downfall. 

One was to revive Guilliman – a feat that Cawl could now accomplish thanks to the aid of Eldar technology. Using a suit of advanced Power Armour that regenerated the wounds inflicted during his last battle, Guilliman was restored to health just as Abaddon’s Black Legion forces invaded Macragge. 

 Reborn, Guilliman swiftly drove off the Chaos invaders before grappling with the events that had occurred during his ten thousand year absence.