Alpha Legion - Death and Rebirth: Meanwhile, Archamus, Rogal Dorn's Master of the Huscarls, was eventually able to deduce the Alpha Legion's true intentions -- to sabotage the Imperial astropathic monitoring network centred on Hydra. During the fighting, Archamus led a Huscarl contingent in a daring assault upon the Alpha Legion force led by Primarch Alpharius himself. In the subsequent fighting, the Huscarls were killed and Archamus was left mortally wounded. All seemed lost, as the battle seemed to swing in favour of the Traitor forces.But miraculously, the Imperial Fists' mobile star-fortress Phalanx, led by Rogal Dorn himself, arrived with a massive Imperial Fists fleet in tow. They were boosted by additional reinforcements from the Armada Imperialis. Utilising the competing gravity wells throughout the Solar System, the Imperial Fists fleet managed to arrive quickly by slingshotting itself towards Pluto. The Imperial Fists reinforcements immediately pacified the captured guns on Kerberos, as Rogal Dorn led his Huscarls in an assault upon the astropathic chamber on Hydra.Dorn and his remaining Huscarls teleported directly into the locked chamber, and soon joined battle against Alpharius and his elite Lernaean Terminators. Archamus looked on helplessly, as the two primarchs fought one another in an epic life-or-death duel. When Alpharius was about to impale Dorn with his Pale Spear, the now-dying Archamus attempted to intervene in the battle after spotting Alpharius was primed for a killing blow, however his seax rebounded harmlessly off of the spear's haft.Archamus was unaware that Dorn had anticipated the strike and stepped in to take the blow with his shoulder to pin Alpharius in place, then grabbed the spear with one hand and using his deadly Chainsword, Storm's Teeth, Dorn sliced through Alpharius' wrists, severing his hands from his arms, before slashing his brother across the chest and impaling him with his own spear. Finally, Dorn finished off Alpharius with a deadly chop of his mighty Chainsword into the top of his brother's skull.With the death of their primarch, the Alpha Legion fleet withdrew and retreated from Pluto. At the moment of his twin-primarch's death, Omegon sensed the demise of his sibling and became distant. Shortly thereafter, he was notified that the Warmaster demanded that Alpharius speak with him so that he could know the status of the Alpha Legion's attack on the Sol System. Like he had done many times before, Omegon immediately took upon himself his brother's name, and permanently assumed the mantle of sole primarch of the XX Legion.During the remainder of the Heresy, the Alpha Legion split off from the main body of Horus' forces early and did not attack Terra with the majority of the Traitor Legions, instead embarking upon a series of delaying actions in an attempt to hold potential Imperial reinforcements for the defence of Terra in place.They also engaged in smaller actions, defeating a White Scars force on Tallarn and a Space Wolves unit at Yarant. Despite their contributions, however, Horus was ultimately defeated by the Emperor aboard his battle barge at the end of the Siege of the Imperial Palace on Terra.