Primaris Space Marines are the next step in the evolution of the Emperor’s Space Marines and were formed for Mankind’s darkest hour.
Via our good friends at Lexicanum
Primaris Marines are larger and more physically powerful than their standard cousins in addition to having a more stable gene-seed. These mighty warriors have appeared in the closing days of the 41st Millennium following the Thirteenth Black Crusade and resurrection of Roboute Guilliman. Primaris Space Marines have been genetically altered by Archmagos Dominus Belisarius Cawl to be bigger, faster and stronger than their Space Marine brethren.
The seed of their creation lies in the aftermath of the Horus Heresy, when the Primarch Roboute Guilliman charged Archmagos Dominus Cawl with creating a new legion of warriors that would aid the Imperium in its next darkest hour. This task would take ten thousand years to complete and now during the Thirteenth Black Crusade, when the Imperium is poised on the brink of annihilation, the Primaris Space Marines have been unleashed to fight against the Chaos hordes of the Despoiler.
Tens of thousands of Primaris Marines were secretly created by Cawl over the millennium, while many more were made after the rebirth of Guilliman. Many of these were produced and held in stasis within the Zar-Quaesitor, Cawl’s flagship. Half of the original batch of Primaris Marines were used to form the new Chapter’s of the Ultima Founding.
The rest were gathered into great armies known as the Unnumbered Sons which would be gradually reduced over the course of the Indomitus Crusade through the reinforcement of existing Chapters scattered throughout the Galaxy. Roughly 94% of existing Space Marine chapters came to accept Primaris Marines within their ranks.
Though they are a step removed from their brothers, the Primaris Space Marines still bear the gene-seed of their Primarchs, and some dissenting voices worry how this new type of warrior will react with the known genetic quirks and flaws of some of the more unusual Chapters. Cawl collected genetic samples of all twenty of the original Primarchs, including those deemed lost to history. However Guilliman made clear that only Primaris Marines from loyalist Legions were to be produced.
The Primaris Marines differ from their standard cousins thanks to three additional Implants not found in the latter: Sinew Coils, the Magnificat, and the Belisarian Furnace. The implants allow the Primaris Marines to be larger and physically stronger than previous generations of Astartes. In addition, their gene-seed is far more stable and only has a 0.001% genetic deviancy per generation, avoiding the severe instability problems seen with Chapter’s such as the Blood Angels and Space Wolves.