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. The Alpha Legion's primarch was named Alpharius, and was actually one of the identical twin Primarchs Alpharius Omegon, two brilliant and secretive sons of the Emperor, one of whom was believed slain at the Battle of Pluto during the Horus Heresy by Rogal Dorn of the Imperial Fists and the other reportedly after the end of the Heresy by Roboute Guilliman, the primarch of the Ultramarines. 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. The Alpha Legion was once the XX th Legion of Astartes created during the First Founding by the Emperor of Mankind to carry out His Great Crusade to reunite all of Humanity in a new golden age under His rule.

The Alpha Legion is the Chaos Space Marine Traitor Legion about whom the least is known.

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