The Battle of Pluto was one of the earliest engagements of the Solar War, when the Traitor Legions, under Warmaster Horus, began encroaching upon the Sol System and the Imperium of Man's Throneworld of Terra during the later years of the Horus Heresy in 010.M31. This campaign began as infiltration forces and probing raids from without, and Dark Mechanicum attacks and attempted break-outs from the besieged Traitor forces on Mars from within, to test the defences of the Throneworld of humanity.
The Alpha Legion carried out multiple diversionary attacks of sabotage and preparation, which plunged the Sol System into complete chaos. Their true target, however, was the fortress moon of Hydra which orbited Pluto. Here, the Primarchs Alpharius Omegon would be confronted by Rogal Dorn, the Praetorian of Terra, in an epic duel that would go unrecorded in official Imperial history but would result in the death of one of the twin Primarchs.
As the Horus Heresy raged throughout the galaxy, Lord Dorn fortified the defences of the Throneworld and the Sol System for the Traitors' inevitable push on Terra. Spheres of defence were created that extended from the literal walls of the Imperial Palace to the darkness beyond the edge of the Sol System. On this outer wall stood the Praetorian of Terra, Rogal Dorn, and his Imperial Fists. All the defences and forces within each Sphere were commanded by a senior Imperial Fist raised to the rank of Lord Castellan by Rogal Dorn.
The First Sphere was the outermost, and came under the command of Sigismund. The Second was commanded by Halbrecht, while the Third, under Efried, enclosed the Martian battle sphere and had the traitorous Dark Mechanicum Tech-priests present on the Red Planet bottled up. Terra was marshalled by Rogal Dorn himself. The Imperial Fists forces in the Solar System were divided between each of these commands, along with thousands more warships, void defences and millions of mortal troops.
Separated by millions of kilometres, Vox signals sent between the forces assigned to each sphere took solar hours to reach each other. When the Traitor invasion finally came, word would pass by a single astropathic flare, a psychic message that moved as fast as thought that said that the Traitors had come to the walls of the Solar System -- "Fire on the mountains".
The Solar Campaign
With its network of operatives and bondsmen- and -women, it is believed that even in the state of civil war that the Imperium had fallen into, the Alpha Legion had the means and opportunities to transfer materiel and personnel between sectors, including the Segmentum Solar and the strategically vital Sol System, whose defence had been entrusted to the Imperial Fists and their Primarch, Rogal Dorn, the newly proclaimed Praetorian of Terra.
Under the tireless efforts of the Imperial Fists, the Sol System had become a fortress, each strata of the Solar System being turned into a perfectly organised defence zone to break the Warmaster Horus' eventual assault. Given the Alpha Legion's need to prove its own superiority, it would not simply suffice to break open Terra's outer defences. To humiliate the VII Legion, they would have to infiltrate the most secure location in the entire galaxy -- the Imperial Palace itself.
Activating assets that had been dormant and hidden on Terra for several standard decades, the Alpha Legion would eventually succeed where all others had failed before, infiltrating several agents within the Imperial Palace and especially the Investiary, where great statues had been erected to commemorate the Great Crusade's greatest generals -- the Primarchs of the twenty Space Marine Legions. Two plinths had stood empty for a long time, the statues adorning them destroyed when their respective owners were cast to oblivion and their history erased -- the Lost Primarchs. But the nine statues of those Primarchs having turned against the Emperor had merely been covered up.
This was the Alpha Legion's target. Infiltrated Legionaries succeeded in penetrating the Investiary and destroyed all the statues except two of them -- those of Alpharius and Rogal Dorn. Intended as both a challenge and a message, this feat was deliberately kept secret from the other organisations within the Imperial Palace. Even the Regent of Terra, Malcador the Sigilite, and the Emperor's own bodyguards, the Legio Custodes, were forbidden to enter the Investiary and witness the shaming of the Imperial Fists.
By what means the Sigilite still discovered the presence of the Alpha Legion on Terra remains a mystery, but Rogal Dorn was adamant that he would deal with the treacherous XX Legion in due course. Shortly after this act of sabotage, an Alpha Legion fleet, led by Harrowmaster Kel Silonius, attacked the Sol System's outermost defences, and managed to even capture several of Pluto's moons, which constituted the heart of the outermost defence perimeter. What truly occurred there remains a well-guarded secret, one only kept by Rogal Dorn himself and his Huscarl-bodyguard.
Battle of Pluto Begins
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 Solar 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 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.
At the same time, 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 Terran 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 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 ships 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.
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.
- Praetorian of Dorn (Novel) by John French