"The Imperium is riddled with corruption and hatred. We have made sure of it."

"Hydra Dominatus!" or "For the Emperor!" (Believed to be used as a mockery; but may actually indicate the Legion's true loyalty)

Alpha Legion is the Traitor Legion about whom the least is known. The Alpha Legion was once the XXth Legion of Astartes created during the First FoundingEmperor of Mankind to carry out His Great Crusade to reunite all of humanity in a new golden age under His rule. They are experts in infiltration, covert operations, misdirection and diversion tactics, and their armies contain many Chaos Cultists and other mortal agents in addition to regular Heretic Astartes.

Primarch was named , and was actually one of the identical twin Alpharius Omegon, two brilliant and secretive sons of the Emperor, one of whom was reportedly later killed after the end of the Horus HeresyRoboute Guilliman, the PrimarchUltramarines. Despite its seeming allegiance to Chaos, a closer scrutiny of the Alpha Legion's known history indicates that their seeming service to Chaos Undivided might actually be the greatest deception they have played upon the Imperium.
This is said to have caused acrimony between Alpharius and Roboute Guilliman when the Alpha Legion was tasked with conquering the world of Tesstra Prime. The XXth Legion .