The Sangprimus Portum is an ancient Imperial artefact that contains the potent genetic material originally harvested from the Primarchs themselves to create the ancient Space Marine Legions. This artefact was entrusted to Archmagos Dominus Belisarius Cawl by the Ultramarines Primarch Roboute Guilliman shortly after the Second Founding in the early 31st Millennium.
Nearly every Space Marine created since the First Founding possesses nineteen specialised organs derived from this gene-seed. The Primaris Space Marines, however -- originally engineered by Cawl on the orders of Roboute Guilliman -- are implanted with a further three additional organs. It was this device that allowed for the breakthrough and resulted in a new breed of Adeptus Astartes. Due to Cawl's interpretation of his orders and the millennia-spanning labour of his task -- during which Guilliman was injured and suspended in stasis for ten thousand standard years -- the secrets of these new Primaris organs were not released until the end of the 41st Millennium.
The Primaris Space Marines were presented to Guilliman after his resurrection in 999.M41, and were deployed en masse in the Ultima Founding, bastions of hope in a galaxy torn apart by the Great Rift that formed in the wake of the destruction of Cadia during the 13th Black Crusade. This massive Warp rift tore through the Materium and spread across the galaxy from the Eye of Terror to the Hadex Anomaly in the Eastern Fringe. The Indomitus Crusade saw the Primaris Space Marines spread throughout the galaxy as part of Guilliman's strike against the birth of a new age of darkness, to lend their strength wherever it might be needed.
By the first century of the 42nd Millennium, Cawl became increasingly detached from the wider galaxy, instead focusing on his quest to create and perfect the Primaris Space Marines. As part of that initiative, he sought to persuade Guilliman to allow him to utilise gene-seed from the forbidden genetic lines of the Traitor Legions and even the long-lost II and XI Legions which had also been extracted from the Sangprimus Portum.
These dangerous and long-thought destroyed genetic lines all displayed no sign of degradation or incidence of unwelcome tendencies within the recipients. All was well according to Cawl. The Archmagos was so satisfied that he repeated his request on multiple occasions that these gene-lines be put into full production, and their recipients be allowed to serve the Imperium as the Emperor had intended. Cawl believed that the characteristics of Guilliman's lost brother Primarchs were too valuable to discard.
Cawl argued that the Emperor's original schema of warriors bred to specific purposes was sound, and should be exploited. Under the circumstances prevalent in the Imperium after the birth of the Great Rift, the Imperium found itself operating with only half the weapons potentially available to it. Cawl believed that by putting the remaining eleven augmented Primaris Astartes gene-lines into production this would allow for far greater tactical and strategic flexibility for Space Marine forces, particularly if they worked in concert.
Cawl passionately argued that the science was not at fault. The Lost Legions' Primarchs had been. Chapters from even Guilliman's own gene-line had also fallen to corruption in the past millennia, yet he had not censored them. But Guilliman emphatically refused the idea of bringing back the sons of the Lost Primarchs, and ordered the Archmagos to not pursue this line of research any further, even as he knew that Cawl would most likely take it upon himself to pursue such heretekal lines of inquiry on his own.
Guilliman was no fool, for he knew that all Magi of the Adeptus Mechanicus hungered for knowledge. When they had it, they could rarely refrain from using it. On this particular matter, Guilliman did not trust Cawl one whit. For all his usefulness and his desire to save Mankind, Guilliman could foresee a time when Belisarius Cawl might become a problem for the Imperium as he repeated the mistakes of those of the ancient Mechanicum who had once followed Horus.
- Codex Adeptus Astartes - Space Marines (8th Edition), "Gene-Seed," pg. 10
- Warhammer 40,000: Rulebook (8th Edition), "Making of a Space Marine - Primaris Space Marines," pg. 59