In the fictional universe of Warhammer 40,000, the Primarchs were engineered superhumans created by the Emperor to command his future Legions of Space Marines. Originally twenty in number, they were said to have been created using the Emperor's own DNA. Of the 20 Primarchs, 18 are known while two remain mysterious. Their genetic material was also used to form the basis of the Legions they would command.
All were created in genetic laboratories hidden on Terra, and were almost completely matured before their incubation pods were scattered across the galaxy. The cause of this is unknown; the forces of Chaos, the Emperor and the Primarchs themselves are claimed in varying tales to be responsible for snatching the infant Primarchs' incubation cradles from Earth and dispersing them through the Warp.
According to the novel False Gods, The Emperor might have allowed the forces of Chaos to scatter the Primarchs across the Galaxy for purposes unknown. Also, according to Chaos, the Emperor might have procured the power needed to create the Primarchs from the Chaos gods via unknown means. As the Great Crusade progressed, the Emperor was reunited with each of them as his armies re-established contact with the worlds on which the Primarchs had come to rest. Most had, in the intervening time, risen to positions of power or infamy as a result of their remarkable physiology and abilities. At least two had been subject to mutation; Sanguinius, who had angelic wings and was reported to be psychic; and Magnus, who had bright red skin and was widely known to have formidable psychic talent.
The Primarchs were the twenty genetically-engineered "sons" of the Emperor. The Emperor used his own DNA in their creation, and they were designed to be far superior to the average human: immensely larger, stronger, hardier, faster, and more intelligent. They were also incredibly charismatic, as their main role was to be generals and leaders of the Imperial military. In addition, their genetic material was used to create their own "sons," the Adeptus Astartes. But while the Space Marines are known to be better in every aspect compared to baseline humans, they are but pale shadows compared to their gene-sires who are akin to demigods.
During their creation, a warp vortex caused by the Chaos Gods collected the infant Primarchs' incubation pods and scattered them across the galaxy. Sensing that the Primarchs still lived, the Emperor of Mankind created 20 Space Marine Legions, each based on the DNA of a single Primearch, to go into the galaxy to find their lost leader. Each Primarch landed on a long-lost planet inhabited by humans and would be strongly affected by the planets' cultures, their interactions with their father upon reuniting with them, their relationships with their legion and siblings, along with their exposure to the Warp when they were scattered. After the end of the Horus Heresy, half of the Primarchs and their legions turned Traitor and were forced to flee the Imperium into either going underground or into the Eye of Terror. Meanwhile, surviving traitor Primarchs became Daemon Princes, immortal Chaos Champions who achieved immortality and Daemonhood but at the expense of being restricted to residence in Chaos realms most of the time. The rest of the Primarchs and their legions were divided up into thousand man chapters to prevent another intergalactic civil war with the surviving Primarchs vanishing from the scene or being incapacitated shortly afterwards.
The overall order of the rediscovery of each Primarch by the Emperor is as follows...
Horus
Leman Russ
DELETED FROM IMPERIAL RECORDS (canonical reason for absence)
Ferrus Manus
Fulgrim
Vulkan
Rogal Dorn
Roboute Guilliman
Magnus the Red
Sanguinius
Lion El'Jonson
Perturabo
Mortarion
Lorgar
Jaghatai Khan
Konrad Curze
Angron
DELETED FROM IMPERIAL RECORDS (canonical reason for absence)
Corax
Alpharius Omegon
Horus Lupercal was killed by the Emperor of Mankind during their battle on Terra with his soul obliterated to prevent him from returning as a Daemon Prince. His body was misplaced several times before being destroyed by his second, Abaddon, to prevent it from being used by his enemies.
Magnus rose to the rank of Daemon Prince of Tzeentch.
Angron rose to the rank of Daemon prince of Khorne.
Mortarion rose the rank of Daemon Prince of Nurgle.
Fulgrim rose to the rank of Daemon Prince of Slaanesh.
Lorgar rose to the rank of Daemon Prince of Chaos Undivided.
Perturabo rose to the rank of Daemon Prince of Chaos Undivided.
Konrad Curze was assassinated by the Callidus assassin M'shen. He allowed his assassin's infiltration, apparently wanting to die to justify himself and his convictions. His head was taken by M'shen as proof of her kill.
Alpharius apparently was killed by Roboute Guilliman after the Horus Heresy. The source which reported his death may have been manipulated. It is not known if Alpharius or Omegon is the Primarch killed in the report. Due to the report's unreliability, neither may have been killed. To make matters more perplexing, Rogal Dorn apparently slew Alpharius on Pluto before the Siege of Terra but kept his death a secret to deny him any recognition to him or his ideology. Meanwhile, Omegon took his brother's place in public as Alpharius upon being summoned to meet Horus before the Siege of Terra.
Ferrus Manus was killed at the Drop Site Massacre at the start of the Horus Heresy by Fulgrim. His skull was offered to Horus but was ultimately returned to the Iron Hands after the Horus Heresy.
Sanguinius was slain by Horus after refusing to join him during the Battle of Terra at the end of the Horus Heresy. His body was placed in a stasis crypt on Baal afterwards.
Lion El'Jonson disappeared after a defeat in battle against his corrupted foster father, Sars Luthor, before placed by the Watchers in the Dark deep within "the Rock" , the biggest remaining piece of Caliban and current home base of the Dark Angels where he has sleep away for millenia. Only the Emperor of Mankind and the Watchers know of this secret with even his "sons" amongst the Dark Angels unaware of his fate.
Jaghatai Khan disappeared while pursuing the Dark Eldar into a Webway portal after they had managed to raid Mundus Planus. There are rumours that he fights there still, lost in the twisting paths of the Webway. After nine thousand years it seems highly unlikely, but not impossible given the Webway's relatively slow passage of time as a Warp construct.
Leman Russ disappeared in the Eye of Terror with his personal retinue. It is rumoured he either set out to kill Magnus or to find a cure for his father's grievous wounds. His final words before his disappearance is that he would return for the 'Wolftime' (the final battle).
Rogal Dorn boarded a Chaos cruiser during the First Black Crusade alongside a company of his legion and allegedly died fighting. A skeletal fist is allegedly all that the Imperial Fists were able to retrieve.
Roboute Guilliman was slashed across the throat with an envenomed blade by his former brother Fulgrim after the Horus Heresy. His dying body was perfectly preserved in a stasis field at the heart of the Temple of Correction for ten thousand years. It was also rumored that wound is slowly healing but denied by others. Recent media has him healed and brought back from the brink of death by Mechanicus and Eldar intervention. He is the only known loyalist primarch that is confirmed to be active and is now preparing the Imperium's defenses from Terra after reclaiming his seat within the High Lords of Terra.
Vulkan disappeared without a trace. He was last seen locked in mortal combat with a massive ork known as the Beast during the War of the Beast (the greatest ork threat to the Imperium in recorded history in the 32nd Millennium) before their battleground was engulfed by explosion of a giant reactor. The Beast survived to be killed later on but Vulkan disappeared. The Salamanders believe that their gene-sire will return once they have found all of his nine artifacts that he hid away with five of them having been found. Given his regenerative abilities, immortality, and self-made prophecy that he would return in another era before his battle with the Beast, his survival is a high possibility.
Corax was racked by guilt and shame. In order to rebuild quickly the strength of his legion from the Drop Site Massacre on Isstvan V, Corax accelerated the growth of the gene-seed organs with the use of fresh genetic material, producing more Space Marines, but this process also deteriorated the gene-seed (with covert sabotage from the Alpha Legion), causing many to become hulking monsters. Riddled with guilt over what he had done, Corax administered euthanasia to all of them after the Horus Heresy and then locked himself away within his sanctum, the Raven's Tower. Exactly one year later he left his tower, haggard and gaunt, and took a small shuttle craft with the words Never more. It was last monitored setting a course for the Eye of Terror.
Almost nothing is known about the two unknown primarchs. Referred to enigmatically as "the forgotten and the purged," all that is known is that they were intentionally erased from Imperial records before the Horus Heresy some time before Corax and Alpharius were recovered due to some transgressions, severe mutation, or catastrophes while their legions were rumored to have been eradicated, absorbed into the Ultramarines, or a combination of both. Outside of the game universe, the Games Workshop explains that their absence is a reference to the lost legions of Rome, and is an opportunity for tabletop gamers to create their own custom Space Marine Legions. Before Games Worshop explicitly states that the two universes were separate, many people say that Sigmar from Warhammer Fantasy is one of the lost primarchs and still continue to do so.
