For most guardsmen, the Carpathian Tin Ghosts were merely a story, told round a standard chemfuel survival and ration heating blaze. They were infamous, and, out of earshot of the comisariat, horrifying.
Said to be harbringers of rad-pollution strong enough to make flesh fall from bone over the course of a few weeks, legends said any who met the gaze of their glowing red optical sensors would die withing 24 terran hours.
Others, trying to scare fresh conscripts, told of their radioactive output being so potent that at extreme short range, it could be picked up as a steady, low clicking on standard issue coms equipment - but by the time any command unit detected it, it was far too late to save the platoon from the creeping poisoning of degrading cells and the boiling agony of mircowaved organs.
The glee with which a more experience radio operator could make his receiver suddenly emit these same low clicks, and subsequently make every guardsman within earshot scramble for his lasrifle with all the dignity and bravery of a snotling fleeing a particularly hungry squig was a brutal contrast to the reality of the Tin Ghosts of Carpath
Their role as reconnaissance and linebreaking meant that often only space marines would see them for more than a few seconds at a time, as fleeting units silently picked their way through the rubble of the battlefronts of the 41st millenium.

However, of all these rumors, only the most horrifying were verifiable, and of course, completely classified beyond the higher echelons of the Adeptus Mechanicum. Referred to in official documents only as the D.A.K. 16.10.16, they had indeed originated from Carpath.

A small moon, someway into the Ultramar system, Carpath 9 was a forgeworld in the hands of the Astra Militarum. Requests went in, Chimera chassis and troops came out. When the hivefleets arrived, Carpath was in disarray. First they ran out of turrets - troops ferried to the front with little in the way of defense, beyond the rifles in their hands. Then as the tracks for the tanks ran out, hollow shells of Chimeras were dragged into place as makeshift defensive walls. Finally, as so often it happened throughout the imperium, the Guard ran out of bodies to throw at the enemy
As bioforms spilled through the sky, and gaunts boiled in the underhives below, the planetary governor, Advisor General Mourn sent a desperate plea out to the Adeptus Mechanicus, promising them acess to all the manufactorums and stcs they could salvage from the planet, in exchange for his safe transit off Carpath.
By the time they arrived however, all that was left was an orbital hospital ship, filled with the broken bodies of what remained of the planetary defense force. Mourn was nowhere to be found.
However, in the cold, glassy sensors of a tech priest - resources are resources, and squander is heresy.
The few remaining conscripts were vivisected into killing machines, their name completely altered, reduced to the point of anonymity, representing their role at it's most minimalist level, to avoid association with the humiliation suffered at the hands of the tyranid invaders.
Considered shamed, and completely expendable, they are prohibited from donning the red robes of mars, and sent on forward missions of such extreme danger that few other admech maniples would ever be assigned them.

Despite rigorous neuro-scraping, the battle servitors of the D.A.K. 16.10.16 fight with the ferocity born of humiliation, and their high rad-output symbolizes their contempt for the frailty of living flesh, and the distance they keep from humanity in penance for their own failures as men.
(I lifted the DAK161016 moniker from a lame, but apocalyptic creepypasta from a few years back that I got a laugh from - the 161016 is like a reference to Revelations and also the coordinates of tel miggado - Armaggedon to you and I and the ancient greeks, while the DAK acronym is delightfully flexible, my favorites being 'double above knee (amputation)', 'dead at keyboard', 'deny all knowledge' and 'does anybody know')