Alpharius Omegon, Master of the XX Legion, the Alpha Legion.
As the last Primarch found, Alpharius' first action was to secretly place his twin brother Omegon within the Legion. Due to Alpharius' strong disgust for truth, the warfare of his son was based upon lies and deception. The biggest secret, which he kept exclusively within the Legion was that the commander of the Effrit stealth squad was in fact Omegon, his twin brother. In battle, Alpharius was relying heavily on his maxime of deception, for his sons tried to look exactly alike with their primarch, making it impossible for most to tell apart the primarch from a regular astartes of the XX Legion. Therefore, the two primarchs were depicted as two halves of a whole, being the primarch himself by being the two separated beings in two different bodies, alike, yet not identical. In the aftermath of the Horus Heresy, Alpharius met his blade with the famed Roboute Guilliman, Primarch of the XIII Legion, the Ultramarines, and died after a furious duel. Believing himself victorious, Guilliman thought that the problem which the XX Legion would have potentially caused after the Horus Heresy was deleted of his plans. However, he would be proven horribly wrong, since Omegon would still be alive and take on the mantle of his brother. This is just a wild assumption, but it seems like the only logical consequence to take since Omegon was a handy commander himself due to his nature as a primarch, and it also fits strongly with the current methods of warfare of the XX Legion, which is based upon stealth and the usage of psychic disruption on a large scale, even dismantling entire space marine chapters. Consequently, it is highly possible that Omegon, despite the death of his brother, is still in command to fight.