Board Thread:Warhammer 40k General Discussion/@comment-46.217.88.194-20171117203424/@comment-8397818-20171117205952

Rogal Dorn killed the real Alpharius in the Battle of Pluto. Then Omegon took Alpharius' name as his own. After the Horus Heresy, Roboute Guilliman killed the primarch of the Alpha Legion (logically speaking, it was Omegon taking the role of Alpharius).

It is hard to believe that the Ultramarine's and the Imperial Fists' primarchs fought someone who was not their brother. I think that it is safe to assume that the Alpha Legion was left without Alpharius or Omegon.