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