Okay, time for another primarch Bio, this time I do a traitor, a nasty, evil traitor.
Legion Name: Alpha Legion (real original, when you consider the name eh?)
Homeworld: none, was found in space
Alpharius was found by Horus, who, by this time was already Warmaster and leading humanity against their enemies. Alpharius was the last Primarch to be found.
One day, Horus and his fleet came across a fleet of human pirates who attacked them. The pirates technology was inferior to Horus’ but their amazing tactics allowed them to almost win; almost.
The fleet pursued them into an asteroid fleet and Horus boarded one of their ships with a bodyguard of Astartes warriors. The first enemy they encountered was shot at as usual, however, he dodged the bullets and shot down the Astartes warriors, then charging at Horus. However, before they met in combat they realized that they were brothers; the last of the Primarchs had been found.
Alpharius was sent to Terra to meet the Emperor and given control of the 20th Legion, whom he named the Alpha Legion (as in, the best, I presume). However, some of the older, more elitist Primarchs (Roboute Guilliman to be precise) told Alpharius that, no matter how hard he tried he could never claim as many victories as the Ultramarines.
So, incensed by this comment, Alpharius decided to prove Guilliman wrong. Though he could never win as many victories as the Ultramarines (one of the largest legions of space marines, not to mention Guilliman was the Imperium’s second greatest tactician at the time). Instead, Alpharius decided to focus on quanilty, during one campaign he actually let the defenders dig in, only to have ninety percent of their men die in a week, losing barely a handful Alpha Legionnaires in the process.
However, his desire to do so only brought more scorn from the other Primarchs, who called him a fool for purposely pursuing dangerous and challenging missions.
Then the Heresy came. It wasn’t really surprising the Alpharius sided with Horus, he barely knew the Emperor, and hated Guilliman, a staunch Loyalist, in fact, it is probably most likely that this is the only reason that Alpharius sided with Horus, to prove to Guilliman and the rest of the Primarchs that he was their equal in combat, if not better.
The Alpha Legion fought against the Loyalist Forces throughout the campaign, on Holy Terra, and the distant reaches of space, in the end, they were forced to retreat, but not before they had caused serious damage to the Loyalist Space Marine forces.
Guilliman, in one of his final campaigns tried to break the Alpha Legion, only to be defeated, though he killed Alpharius, they chain-of-command was prepared and fought on, until Guilliman was forced to retreat.
To this day, the Alpha Legion is a thorn in the side of the Imperium. They dwell in hidden bases outside of the Imperium and in secret cults in the darkness of Imperial Hive cities. Though they have been supposedly destroyed three times, they have always reappeared.
Though Alpharius is dead, his legion lives on, and perhaps this is the greatest testament to his skill, he created an unstoppable army, one who for ten thousand years has fought against the Imperium from within it, and has never, and perhaps will never, be truly defeated.