Primarch: Culain MacTurson
Symbol: Two Gears Interlocking, Lightning Bolt from where they connect
Colors: Ethereal Green, Gunmetal Gray, Leather Brown
Capital World: Ilmarinen
Speciality: Mechanized Warfare, Heavy Weaponry
Battlecry: "Strength of Men, Strengthened by Metal!"The X Primarch
Taciturn and often described as humorless, Culain MacTurson is the middle of the trio of Primarchs that the Emperor created to represent mankind's relationship to technology. Whereas Octaviar Perturabo held perfect knowledge of technology and Ogadin Vulkan was the incarnation of mankind's ingenuity and creation, it is the Xth Primarch that is the synthesis of humanity and the soul of the machine.
Logical to a fault, his soul was created to be a natural fit to commune with any machine spirit, no matter how powerful. Though a fearsome fighter with only his natural talents, Culain truly shines when he is in communion with a machine. Be it a Baneblade, a Gloriana-class flagship, or even theoretically a Titan, there is no one else in the entire Imperium save for perhaps the Emperor himself that can coordinate with technology like Culain can. The greatest example of this is seen in his choice of weapons. Whereas originally the Xth Primarch used a massive claymore, as he became more able to attune machines to his will, two powerfists and four mechadendrites became his implements of choice. With a plethora of weapons he is able to choose from for any circumstance on the battlefield, the walking armory that is Culain MacTurson is capable of unleashing destruction on a scale that few other Primarchs are able to match.
His gift from the Emperor is a Dyson Bottle. A relic from the Dark Age of technology, the Dyson Bottle is a miniaturized Dyson Sphere capable of unleashing enough power to provide plentiful energy for an entire sector. Though capable of channeling incredible power into his weapons, as he did when he famously killed the Beast Ork Maga da Zapboss, Culain most often uses his gift at one of the various Forges he uses to craft other weapons and utilitarian tools in service to the Imperium. With such a potent source of power and machines capable of using it, there are no better items forged in the Imperium of Man than by Culain MacTurson, save perhaps those on Saturn or Terra itself.
History of the Legion
The first candidates for the Xth Legion came from the ever-battling clans of Old Albia, a culture known for their brutality as well as respect for prowess. From those radiation soaked wastelands, came some of the most reticent warriors in all twenty legions. When called upon, they would unleash the full spectacle of their heavy weaponry, bombarding enemy locations with artillery strikes that would sometimes last for months on end. Though surrender was eventually offered, rarely was it accepted. The Xth Legion would continue the onslaught until none were left alive to resist the Imperium.
It was for this reason that Culain and his sons were rarely called upon to take part in the liberation of Terra. They were sent to destroy the irredeemable and erase all memory of their existence. This made them one of the Emperor's favorite tools for eliminating enemies, but the early propagandists in the Imperium had little use for their tedious and mechanical style of warfare.
Their pattern of warfare continued into the Solar Crusade, where the Xth Legion busied themselves with destroying dangers to the Imperium, not rescuing the lost dredges of humanity. Their task was the complete and utter destruction of the burgeoning Space Hulk drifting near the edges of the Sol System. That haven for pirates, xenos, and other more unspeakable evils was reduced to atoms by the might of the Xth Legion.
As the Great Crusade began in earnest, neither the Emperor nor Culain saw a need to change the role that the Xth Legion naturally fell into. Instead of focusing on liberating worlds (though they did not shy away from that duty when it arose), they focused on the planets deemed unrecoverable. Infestations of horrific xenos or a human society past the point of redemption, it did not matter to them. All would meet the same fate under constant barrage from the heavy guns of the newly named Stormbringers.

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Imperium Ascendant
Science FictionWe all know the tale, The Emperor of Mankind creates Twenty Primarchs to conquer the Galaxy. Nine Fall to Chaos's taint and Let the Galaxy Burn. What if this was not the case? What if the Ruinous Powers never got their claws into the Emperors Sons...