Omegon

Omegon is the twin of Alpharius and the other primarch of the Alpha Legion, hidden even from most of his own Legion for years. Where Alpharius liked the stage, Omegon liked the machinery under it.

He ran the secret war while his brother took the name in public. After Eskrador the remaining head had a choice: keep the Legion as a weapon, or let it dissolve into a thousand lies. The evidence suggests he chose both.

He has not announced himself since. That is consistent. If the Hydra still has a mind, it is probably his. The Alpha Legion still answers to the name Alpharius in the field. That, too, is probably his idea.