The Emperor had already been fighting on Terra with genetically modified warriors, but in the end they proved hyper-violent and short-lived. This project would be his greatest attempt to create generals to lead these warriors, second only to himself in skill, ability, and sheer power. It was begun in M29 with the aim to create 20 cloned 'children' of the Emperor. The facility to create these Primarchs was constructed on Luna and the Emperor oversaw the project, having them grown in accelerated gene culturing chambers. His scientists watched over the growing fetuses in their chambers, afraid of what would happen if anything went wrong.
According to Ingethel the Ascended, the Primarchs were created by the Emperor with the aid of forbidden sciences and arcane lore derived from the Warp. Eldrad Ulthran also states that the Emperor utilized dark powers to construct the Primarchs. It is speculated the Primarch's were imbued with energies and knowledge that the Emperor gained in a Warp Portal on Molech. However, the Gods of Chaos somehow managed to spirit them away just prior to their maturation with a great Warp vortex and it is speculated that they also managed to tamper with the infant Primarchs. The scattering of the Primarch's may have been the result of a Causal Loop created when Argel Tal and his men were sent back in time inside the Eye of Terror to the Emperor's laboratory, as they were convinced by Ingethel the Ascended to destroy the Gellar Fields protecting the infants from the touch of the Chaos Gods. Almost immediately after the Gellar Field generators were disabled, the Primarch's were sucked into a Warp vortex. Several Primarchs including Konrad Curze, Roboute Guilliman, and even Sanguinius all came to believe that the scattering of the Primarch's and their upbringing on distant worlds had all been engineered by the Emperor.
Scattered across the galaxy, beyond the Emperor's reach, and all landed on different worlds vastly separated from their Father. The Emperor could sense that the infant Primarchs were alive, but was unable to reach them. Therefore he used their previously collected DNA samples to create the Space Marines. The genetic material of each one of the Primarchs was used to found a single Space Marine Legion, twenty in total. The new homeworlds of the infant Primarchs and subsequent experiences on those worlds would influence and mold them to a large extent. There they quickly grew to adulthood and often rose to power, ruling, leading, and helping the inhabitants. As the Emperor found them, he was generally pleased by their actions, and quickly gave them the leadership of their respective legion.
- Codex: Space Marines (4th Edition)
- 1: The First Heretic, Chapter 17, pg.300-301
- 2: Fulgrim (Novel) Chapter 14
- 3: Vengeful Spirit (Novel)
- 4: False Gods by Graham McNeill, Chapters 15-16
- 5: Pharos (Novel) Chapter 24