Alpharius
Alpharius is the mysterious Primarch of the Alpha Legion. We don't have much information on him other than that Omegon, his bro, looks exactly like Alpharius, but we also don't know who Omegon and who Alpharius are, or which one is which. For all we know the entire Alpha legion is Alpharius and Omegon. Hell, they're so secretive they probably betrayed the Imperium to help the Imperium, which helped Chaos beat down the Imperium, which actually helps the Imperium kick Failbaddon's ass. He could probably be the one that's making Abaddon fail so much.
But what the fuck, we don't know.
tl;dr Alpharius is like Anonymous. There's over 3 billion of him and everyone is Alpharius.