Updated the background with info I provided later and also now I can add yet more worldbuilding since I'm not rushing to meet a deadline. I also dun goofed with Atroci's rank: he should be Alpha Primus.
Located just at the edge of the Crassean Sector and close to the only stable warp route taking to the Andala subsector, the Io-IIIb system wouldn't be of much - or any - note weren't for the two habitable worlds in orbit around its yellow star. Named Valtrass I and II, those planets with similar orbits only had attention drawn to them when a tech-degraded human civilization with deactivated Knight Engines revered as monuments of ages past was found to exist on Valtrass II; meanwhile, scans indicated the existence of archeotech caches and valuable minerals on Valtrass I. The Adeptus Mechanicus quickly seized Valtrass I and set upon converting it into a Forge World while the second planet was contacted and made to sworn fealty to the Mechanicus so that they could provide foodstuffs and raw materials for their neighboring planet. The discovery of the templates for the construction of Knight Engines and the quality of Valtrass I's archeotech-enhanced machinery enabled the forge world to begin production of Warhound Titans and also of Imperial Knights with remarkable efficiency. It wasn't long before valorous warriors from Valtrass II competed for the right to pilot a god-machine into battle, leaving behind the lowly title of knight and becoming true Knights and rising up to wield the power their ancestors had long wielded. The inhabitants of the backwater world watched with awe as the most skilled of their number were swallowed into those minor metal deities to do battle, occasionally called upon the red-robed priests of the machines to wage war across the stars. A few of their number were now and then granted the honor to be taught secrets of technology, while others yet were drafted to become the blessed and augmented infantrymen that followed the god-machines small and large beyond the skies. That began millennia ago, and so it remained until recently.
The long-standing order was disrupted with the sudden appearance of Waaagh! Onetoof and consequent increase of ork activity in that area of space. Part of the fleet under command of one of the Warboss' lieutenants entered Io-IIIb to claim all of their dakka months before Andala seceded from the Imperium. The system's Mechanicus petitioned for assistance but didn't cross their arms and wait for it to come; while the astropathic signal was still on its way to the closest military stronghold of the Imperium, monitor-cruisers of the Mechanicus exchanged battery fire with ork kroozers and grossly outnumbered Skitarii fought boyz and nobz in gruesome close-quarters during boarding defense actions. The defensive cordon of vessels was partially bypassed by greenskin contraptions and a number of roks impacted upon the surface, inflicting light structural damage. Ultimately the green menace was purged from the system at the cost of most of the small defensive fleet and acceptable albeit major losses for the Tech-Guard ranks. Most worryingly, the majority of the higher echelons of Valtrass I's Mechanicum had been wiped out during to unfortunately-placed rok crashes and the subsequent surface fighting, prompting fresh batches of priests to be sent in order to replace the losses.
The treason of Basileus Yrine I combined with the threat of Warboss Onetoof and the appearance of Tau was too much of a risk to ignore. The newly-arrived tech-priests stationed in Valtrass I discussed the situation at length and reached a simple conclusion: there was no time to properly equip and train more regiments of full-fledged Skitarii and their presence was necessary for the defense of the system, but at the same time not contributing to the war effort would be illogical. The solution found and put into action was that of raising peasantry levies from Valtrass I and in a rare display of useful logic, the newly-appointed Fabricator-General agreed with the newly-ordained priests' assessment of the situation, meaning part of the manufacturing power of the forge world was specially appointed to equip the levies.
Prefab buildings made planetfall on Valtrass II and atmospheric craft began operations as town yellers spread the word that all able-bodied men and women of fighting age were to present themselves before the red-robed priests, for they were being called upon to fight across the stars and selection would take place in the metal castle that fell from the skies. All skirmishes between warring kingdoms and factions were put to a stop by order of the machine-men, and for several days even the cloak and dagger-fueled politics of nobility had to refrain its hand under direct orders of men who could erase castles off the face of the earth without effort. The number of drafted soldiers-to-be - between young wide-eyed peasants trying to escape the yoke of their lords and ambitious battle-scarred knights - was high enough that more aggressive tactics were unnecessary and those with children could opt out so as to not endanger the future of the planet's population and food production, and of course many of the ruling classes' offspring were granted the option of leave. Regardless of necessity, many daring souls swelled the levy's ranks voluntarily because it was an unique opportunity: they would be taken past the stars to fight in the name of the red-robed priests and their god-machines, given the chance to be much more they could ever become in their homeworld.
The colossal factories of Valtrass I worked cycle after cycle, churning out efficient weapons of war including designs like the "Solo" boltgun, shunned by many traditional adepts of Mars but judged efficient and desirable to meet the necessary quotas. Basic training was swift - part of it scheduled to be completed during voidship transit - and all of the soldiers received a crash-course pertaining to the Omnissiah's blessings on top of hypno-conditioning during their sleep to hasten the learning process. Needless to say, the levied peasants were mystified by all that was being taught to them; they had been whisked away and introduced to a world where cloth armor protected more than plate and blessed red plate shrugged off hits like a block of stone. They were being given strange weapons that roared like the most ferocious beasts and caused destruction on par with siege weapons, and it was all both incredible and frightening. Even the knights, some of them veterans of several bloody wars of nobility, were unable to hide their amazement and one of them was even reputed to have become speechless for an entire day after watching a plasma gun's live-fire demonstration. They believed in the Sky-Emperor, but they also believed in the Machine-God, and the latter's presence was palpable during the training - their adepts of steel walked amidst them, and the few most promising of their ranks were picked to be initiated in His secrets.
The Levy regiment was finally declared fit for shipping after a couple of months, its ranks swollen by a number fit to fill most Guard regiments twice or thrice over. These men and women tasted how it felt to be closer to divinity, but the lingering dread and the certainty they might be forced to fight giants and gods that challenged even the crimson priests and the god-machines robbed some of part of their enthusiasm. Fear, hope, pride and confidence filled minds and hearts as they were taken into the bowels of the giant ships that crossed the black sea of stars - despite all of their doubts and despite the fear of some, one thing was certain: they would wage war away from their world and win - or die trying. Even if they never made it back home, the future generations would remember them still and stories would be told about them, those brave men and women who answered the call of the priests clad in red... As those who walked among the stars.
• Particularities: They are remarkably well-supplied and well-equipped for a regiment mustered from a feudal world - when the Guard's High Command hears of feudal regiments their minds are promptly filled with images of savages in hideshirts wielding primitive weapons, not carapace-armored boltgun-wielding pseudo-tech-guard. The people who make up the ranks of the Levy come from all manner of backgrounds: actually levied serfs, free men, offspring of nobility, knights and more, and they also come from all manner of kingdoms and fiefs, meaning their accents are noticeable and can differ. Most of the Valtrassian Levy's commanding positions are occupied by Skitarii veterans who put the power swords and bolt or plasma pistols used by most Guard officers to shame with their obviously Omnissiah-favored equipment that makes them monsters in the battlefield, from arc-weaponry and transuranic arquebuses to transonic blades and digital weapons.
The Levy soldiers see their superiors with a mix of awe and fear, while a few dream about reaching such heights of personal power. The members of the regiment are likely to have trouble with more religious-minded people thanks to their admiration for the Machine-God (which they see as separated from the Sky-Emperor as opposed to one of His facets) and the god-machines. Morale is overall good thanks to the feeling of empowerment shared by the soldiers, and they have been properly instructed about recovering matériel from the fallen for it is holy and cannot reach the enemy's hands.
• Further details about Valtrass II: The most prolific Knight House installed in Valtrass II is House Valrisian, which has close ties with the Mechanicus and does, albeit rarely, "adopt" promising warriors from "normal" knight orders to test their mettle and judge them whether they are worthy to pilot an Imperial Knight. This practice causes those born in the House's line to try their hardest not to lose their ancestral rights, which means the acceptance of foreign members to the House's training might be a gambit based on that response. The house's colors are rust, copper and light gray in an echo of Valtrass I's techguard.
Only two other Houses have Engines, though in smaller numbers than Valrisian: Bellefort (dark orange and white) and Damasphur (black, light gray and navy). Their smaller numbers ensure the balance of power remains unchanged, and their adoption of a more traditional process to choose their pilots has caused some of the free men to point out it might be the cause of their failings compared to Valrisian - though Valrisian possesses more Engines not only due to better pilots ensuring their survival, but also because their tithing is superior in number and quality than that of their rival Houses.
• Valtrass I Legiones Skitarii's color scheme: rust-color warplate, copper cybernetics and light gray robes with the white symbol of the Legiones - two scissor-like blades side by side emerging from the upper half of a cog containing a small skull - displayed prominently on the back of the hood.