Alpharius Omegon - Battle of Pluto: In order to prepare for Horus' advance towards 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, led by the Praetorian of Terra, Rogal Dorn himself, the 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 down to the bare minimum required to maintain life support in order to reduce their overall heat signature. Thus it took an entire solar 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 were activated throughout the Sol System. These "sleeper" cells had been planted Terran years earlier before the outbreak of the Horus Heresy, and upon activation, performed their primary functions of sowing destruction and carrying out diversionary terrorist acts, which led to outbreaks of panic and anarchy. Acts of murder and sabotage were conducted on multiple planets across 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 Dorn was distracted by the 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, another uprising, instigated by Alpha Legion teams, erupted across the Sol System.The Alpha Legion's primary objective was Hydra, a fortress moon that orbited Pluto and served as 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 of the Imperial Fists, had just 30 warships 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. The captured fortress moon of Kerberos rained down deadly fire upon Charon, Nix, and Styx. Despite facing these 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 intention -- to sabotage 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, commanded by Rogal Dorn himself, arrived with a massive Imperial Fists fleet in tow and additional reinforcements from the Armada Imperialis. Utilising the competing gravity wells of the Sol System, the Imperial Fists fleet managed to arrive quickly by slingshotting itself towards Pluto. The Imperial Fists reinforcements pacified the captured guns on Kerberos, and 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 XXth.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 Vengeful Spirit at the end of the Siege of the Imperial Palace on Terra.