Imperial Fists - Battle of Pluto: In order to prepare for Horus' advance on Terra, the Alpha Legion was charged by the Warmaster to carry out vital acts of sabotage and preparation, which would plunge the Sol System into complete chaos. Though heavily defended by the Imperial Fists Legion, Primarch Alpharius, aboard his flagship Alpha, led his fleet towards the Sol System.He managed to successfully infiltrate the outer defences of the system by putting himself, and the entirety of the personnel within his fleet, into stasis. Meanwhile, the Alpha Legion fleet approached at minimum speed and powered its vessels down to the bare minimum, in order to reduce their overall heat signature. Thus it took an entire standard year for the Alpha Legion fleet to drift towards Sol and successfully reach the outskirts of Terra's defences undetected, as Warp travel had been strictly forbidden.Meanwhile Alpha Legion Sparatoi (augmented and highly adept Human agents) as well as Alpha Legion operative teams activated throughout the Sol System. These "sleeper" cells had been planted standard years earlier before the outbreak of the Horus Heresy, and upon activation, performed their primary functions of sowing destruction and diversionary terrorist acts, causing panic and anarchy.Multiple acts of murder and sabotage were conducted on a number of planets in the system to distract the beleaguered Imperial defenders from the Alpha Legion's true objective -- Pluto -- the lynchpin of the Loyalists' surveillance network within the Sol System.As Rogal Dorn was distracted by these diversionary attacks across the system, Alpharius' fleet, comprised of over 200 vessels, led by the Alpha, struck Pluto and its moons Charon, Kerberos, Nix, Styx and finally Hydra. As the fleet arrived, a further uprising, instigated by Alpha Legion infiltrator teams, erupted across the Sol System.The Alpha Legion's primary objective was Hydra, a fortress moon that orbited Pluto and was a major astropathic monitoring station. Alpharius himself led a team of Alpha Legion warriors in the attack on the moon. Meanwhile, the Loyalist defenders, led by First Captain Sigismund, had just 30 voidships to defend against the massive Alpha Legion fleet.Due to the unexpected and brutal assault, the severely outnumbered Loyalist fleet was badly mauled by Fire Ships hidden among the Alpha Legion fleet, to wreak havoc on the Imperial Fists defenders. The captured fortress moon of Kerberos rained down deadly fire upon Charon, Nix, and Styx with heavy weapons. Despite facing overwhelming odds, the Imperial Fists resolutely stood their ground in the face of the Alpha Legion's overwhelming assault.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 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 Sol System, the Imperial Fists fleet managed to arrive quickly by sling-shotting itself towards Pluto. The Imperial Fist 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 that 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. Dorn grabbed the spear with one hand and using his deadly chainsword, Storm's Teeth, the Imperial Fists primarch 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 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.