Decimus Felix was one of the first Primaris Space Marines who also served as a Lieutenant in the Unnumbered Sons and later as the Equerry of the Lord Commander of the Imperium, Roboute Guilliman, during the Indomitus Crusade. The Lord Commander would appoint him to become the Eleventh Captain of the Ultramarines Chapter, breaking the strictures of Guilliman's own original Codex Astartes-mandated order of battle. As the Indomitus Crusade ended and Guilliman turned to defending his Realm of Ultramar from the forces of Nurgle during the Plague Wars, he announced his intent to massively enlarge Ultramar's territory and restore it to its original size of five hundred worlds. He appointed Decimus Felix as one of Ultramar's new ruling class of Tetrarchs, an ancient rank that the Primarch had decided to restore to prominence. Felix was given rulership over the Ultramaran world of Vespastor, and charged with enlarging Ultramar's eastern expanse by whatever means necessary.
Decimus Felix was one of the first men chosen to become a Primaris Space Marine. Originally hailing from the Shrine World of Laphis in the Macragge System in Ultramar, he was taken as an Ultramarines Aspirant soon after the end of the Horus Heresy in the early 31st Millennium. But instead of becoming an Ultramarines Neophyte, he was instead sent to Mars along with hundreds of thousands of other Aspirants on the orders of Roboute Guilliman to become part of Archmagos Dominus Belisarius Cawl's project to create a new type of improved Space Marine.
Felix spent thousands of standard years in and out of cryogenically-induced suspended animation as he and his fellow Primaris Space Marine candidates were experimented upon as Cawl sought to perfect the Primaris variant of Astartes gene-seed. They were also awoken at various intervals to undergo relentless sessions of hypno-indoctrination. It seemed the cycle of experimentation and psychological conditioning would never end until Roboute Guilliman was unexpectedly resurrected during the Ultramar Campaign of the 13th Black Crusade in 999.M41 by Cawl and his allies among the Aeldari Ynnari faction.
As Guilliman once more assumed the position of Lord Commander of the Imperium, he ordered Cawl to finally unleash the Primaris Space Marines, for the Imperium of Man stood on the brink of disaster as the Forces of Chaos and the xenos threatened humanity at every turn. Guilliman ordered the start of the great Indomitus Crusade into the regions of the Imperium most threatened by the onset of the Noctis Aeterna. Felix was among the nine Space Marine Legion-sized formations of Primaris Space Marines known as the Unnumbered Sons, and eventually rose in the course of the Crusade to become a Lieutenant of one of its companies.
His performance drew the attention of Guilliman himself, who ultimately selected Felix to leave the ranks of the Greyshields and become his Equerry. At the same time, Felix was surprised when the Primarch declared that he was also now a Captain of the Ultramarines Chapter, a position that Guilliman had never given to the other Astartes who had previously served as his Equerries. He was also recognized as the Eleventh Captain of the Chapter, which broke the strictures laid out in Guilliman's own Codex Astartes-mandated order of battle for a Space Marine Chapter.
Felix often was troubled by these promotions, believing himself unworthy of the responsibility which they portended. But Felix performed well in his role as Equerry, and when the Indomitus Crusade ended at the Battle of Rauxos well over a century into the 42nd Millennium, he was among the forces that travelled with Guilliman to aid the defence of his Realm of Ultramar from the offensive of the servants of Nurgle led by the Death Guard Traitor Legion and Guilliman’s traitorous brother, the Daemon Primarch Mortarion. This assault, known as the Plague Wars, had been ongoing almost since the birth of the Great Rift over a standard century before, and the forces of the Ultramarines had found themselves pushed onto the defensive by the time of Guilliman's return to his realm.
Immediately after his arrival, the Primarch convened a meeting with the leaders of the Ultramarines and all of his accompanying forces to hash out a strategy for the defence of the Realm. Guilliman made clear that he not only intended to defeat the forces of the Plague God, but also to restructure Ultramar itself. Guilliman explained that he had made a mistake when in the wake of the Horus Heresy he had consented to drastically shrink the size of Ultramar from five hundred worlds down to a bare handful. It was time to restore the Five Hundred Worlds, and to guarantee that this initiative would be successful, Guilliman recreated the ancient ranks of Tetrarch and announced that his Equerry would be among their number.
Decimus Felix immediately protested that he was certainly not worthy of such a position, but Guilliman claimed that Felix's actions during the Indomitus Crusade had proven over and over that he had all of the personal qualities that being a Tetrarch of Ultramar would demand. After all, the Lord Commander continued, that was why Felix had been made his Equerry, to prepare and test him to take on this vital role. Felix bowed then to his Primarch's wishes. He was assigned to rule the world of Vespastor and charged with enlarging Ultramar's eastern expanse using all the considerable tools at his disposal, reclaiming those worlds from the Noctis Aeterna and restoring them to the Emperor's light.
- Dark Imperium (Novel) Guy Haley, Chs. 15, 20