Primarch

The Primarchs were the twenty genetically-engineered "sons" of the Emperor. Using his own DNA in their creation, the Primarchs were designed to be far superior to even Space Marines: they were taller, stronger, faster and more intelligent. They were also incredibly charismatic and were well suited to their role as the generals and leaders of the Imperium of Man.

The Primarchs were created in a secret underground laboratory on Terra's moon of Luna under the tighest security. However, due to insidious dickery, the forces of Chaos were able to spirit the Primarchs away from the laboratory and scatter them across the universe. Just as planned. There is also some speculation that the Chaos gods tampered with the Primarchs somehow, which would explain the mutations. Either way, the Emperor knew his sons were still alive, but had no clue to where they could be, so he decided to go ahead with his Space Marine project. Using DNA samples from each Primarch before they were abducted, the Emperor created twenty legions of Space Marines; one for each Primarch.

Each of the young primarchs were forced to adapt to the lifestyle on their new homeworld, something which would influence and mould them throught their lives. The primarchs rapidly grew to adulthood and quickly rose to power, often becoming the leaders of their world. As the Emperor crusaded to unify the galaxy he would occasionly stumble upon another long-lost son. When this happened, the Emperor would hold a celebration in honour of this discovery, give the Primarch their legion to command, then tell them to fuck off and start conquering worlds while he fapped to his favoured offspring. The Emperor was a dick like that.

The first Primarch the Emperor discovered was Horus, who would become the Emperor's favoured son. The majority of Primachs somehow recognised the Emperor on sight, immediatly pledging their allegiance to their father. A few only swore allegiance after being bested in a contest. Angron outright refused to follow the Emperor, perfering to die in battle along with his rebels in their fight against the oppressive Not-Romans. The Emperor simply shrugged and abducted his son, leaving Angron's followers to get slaughtered. Angron never really got over that dickmove.

At the end of the period known as the Great Crusade Horus, who had recently been promoted to the title of Warmaster, fell to Chaos and rebelled. It didn't take long for eight other of his fellow Primarchs to join his side against the Emperor, resulting in a full-scale civil war known as the Horus Heresy. Funnily enough, most of the Primarchs who sided with Horus were those who felt that the Emperor had taken a giant, steaming dump on them. Smooth move there, Emperor.

Legion Number	Name	Homeworld	Name of the Legion	Allegiance	Current Status
I	Lion El'Jonson	Caliban	Dark Angels	Loyal	Missing
II	+++Records expunged+++
III	Fulgrim	Chemos	Emperor's Children	Traitor	Possessed/Daemon Prince
IV	Perturabo	Olympia	Iron Warriors	Traitor	Daemon Prince
V	Jaghatai Khan	Mundus Planus	White Scars	Loyal	Unknown
VI	Leman Russ	Fenris	Space Wolves	Loyal	Unknown
VII	Rogal Dorn	Terra	Imperial Fists	Loyal	Deceased
VIII	Konrad Curze	Nostramo	Night Lords	Traitor	Deceased
IX	Sanguinius	Baal	Blood Angels	Loyal	Deceased
X	Ferrus Manus	Medusa	Iron Hands	Loyal	Deceased
XI	+++Records expunged+++
XII	Angron	Unknown	World Eaters	Traitor	Deceased
XIII	Roboute Guilliman	Macragge	Ultramarines	Loyal	Alive, unfortunately.
XIV	Mortarion	Barbarus	Death Guard	Traitor	Daemon Prince
XV	Magnus the Red	Prospero	Thousand Sons	Traitor	Daemon Prince
XVI	Horus	Cthonia	Luna Wolves/Sons of Horus	Traitor	Deceased
XVII	Lorgar	Colchis	Word Bearers	Traitor	Daemon Prince
XVIII	Vulkan	Nocturne	Salamanders	Loyal	Missing
XIX	Corax	Deliverance	Raven Guard	Loyal	Missing
XX	Alpharius Omegon	Unknown	Alpha Legion	Traitor	Fuck knows.

Two Missing Primarchs

Who are the two missing Primarchs, you ask? Sigmar and Samus.

...

Okay, fine. Nobody really knows. There have been a few shitty hints about them in the Horus Heresy books, but nothing really concrete. What we have established is that they got killed for some reason and the existance of their legions were wiped from record. The surviving marines from both legions were then gobbled up by the Ultramarines for some retarded reason. As for the identities of these two Missing Legions? Well, Games Workshop have had a few canidates over the years. In the short story Hell in a Bottle from the novel Into the Maelstrom, a chapter known as the Iron Hearts get fucked over by a Chaos Lord. The short story also mentions that the chapter has a Primarch known as Rubinek. Of course, this was just a huge cock-up on the author's part. Another possible canidate is the Valedictors, a who were mentioned in two early issues of White Dwarf as being a First Founding chapter who saw action against Orks, Eldar and Traitors during the Horus Heresy. However, around a hundred issues later, they were again mentioned as being post-second founding.