The Sangprimus Portum, originally known as the Magna Mater after the Latin name for the ancient Anatolian goddess of fertility Cybele, is an ancient Imperial artefact that contains the genetic material originally harvested from all twenty Primarchs to create the ancient Space Marine Legions.
During the First Pacification of Luna by the newborn Imperium at the start of the Great Crusade in 798.M30, the ruling Selenar High Matriarch Heliosa-54 charged the Selenar gene-witch Ta'lab Vita-37 with safeguarding the Magna Mater from the forces of the Emperor of Mankind.
She managed to keep it hidden for over two Terran centuries.
However thanks to the unexpected intervention of the Gloriana-class Battleship Photep, the flagship of the Traitor Primarch Magnus the Red, Nykona Sharrowkyn was able to hold onto the Magna Mater and hide it with himself aboard an abandoned void station.
Putting himself into suspended animation to survive alone, Sharrowkyn noticed the nameplate of the station read Sangprimus Portum, the name by which the artefact was to be known in later centuries.
Through Sharrowkyn's efforts the Sangprimus Portum eventually came into the possession of the Primarch Roboute Guilliman after the end of the Horus Heresy.
The Sangprimus Portum was employed by Imperial scientists to store copies of the genomes of each of the twenty Primarchs which were more pure than any other known samples.
Guilliman gave the artefact and its precious contents to Archmagos Belisarius Cawl and ordered him to use it to create a new generation of Space Marines.
The artefact and its more pure genetic samples of the Primarchs' genomes would prove to be the crucial ingredient needed to produce an improved template for the Emperor's transhuman warriors -- the Primaris Space Marines.
Nearly every Space Marine created since the First Founding possessed nineteen specialised organs derived from his Primarch's gene-seed. The Primaris Space Marines, however were implanted with a further three additional organs. It was the Sangprimus Portum that allowed for the breakthrough that resulted in this 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 and the dawn of the Era Indomitus.
The Primaris Space Marines were presented to Guilliman after his resurrection in 999.M41, and were deployed en masse in the Ultima Founding. They were 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.
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.
Further Use of the Sangprimus Portum
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 further 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
- Siege of Terra - Sons of the Selenar (Novella) by Graham McNeill, Prologue, Chs. 4, 11