The Pale Throng are a Mutant Cult and anti-Imperial warband active in the Calixis Sector. They are considered one of the most dangerous and destabilizing elements in the region. Their goal is to liberate Mutants and Psykers from the shackles of Imperial oppression, and visit bloody vengeance upon their overlords. Beyond their Mutant liberation platform, they maintain that Mutants and Psykers are superior to normal humans and represent the future of mankind.
They are no Chaos Cult, but are prone to manipulation from external forces, opening the possibility to being used as a tool in a Chaos scheme. They do not act subtlety or with conspiracy. Rather, they are a hyper-violent band of hideously deformed killers and Rogue Psykers whose trademark is the destruction and atrocities against non-Mutants they leave in their wake.
The Pale Throng finds its origins on the world of Tranch, a smaller Hive World. The planet did not purge their mutant population, as is standard throughout the Imperium. Rather, they were used as slave labor for the world's factories and made up a downtrodden underclass. Despite their horrible conditions and oppression by Tranch's PDF, they left the Mutant population largely alone in their lawless settlements deep in Tranch's Hives. This proved a fatal mistake for the Oligarch rulers of Tranch.
Unknown to the Oligarchs, a group that called themselves the Shroud Masters sprung up among the mutant population. Though their origin is unknown, what is known is that the Shroud Masters organized and trained the mutant population to bring down the rulers of Tranch. So it was that the first forces of the Pale Throng were mobilized, taking advantage of the innumerable number of mutants on Tranch. When the time was right, the Shroud Masters unleashed the mutant hordes of the Pale Throng upon the Oligarchs of Tranch. The surprise and massive uprising by the mutant slave class wrought chaos, and within a week the hive world of billions spun out of control and saw millions dead.
The Imperial Guard hastily assembled a counter-insurgency operation drawn from conscripts across the sector, ultimately climaxing in a bloody and infamously brutal conflict known as the Tranch War. The rebellion was ultimately crushed by the Imperial forces, but it was a Pyrrhic victory and most of Tranch lay in ruins. Even the Oligarchs of Tranch were purged by the Inquisition for their ineptitude, which had allowed the war to break out in the first place.
But while the Tranch War is a relatively well-known conflict, the Imperium keeps its darker truth a secret. For the identities of the mutant overlords of Tranch, the Shroud Masters, are beings of great skill and intellect with the power to manipulate the Warp. Fortunately for the Imperium, the Shroud Masters Council imploded during the conflict and saw the Pale Throng fragment into smaller warbands. Yet still, there are several Shroud Masters active, furthering the cause of the Pale Throng independently of each other.
Despite its defeat in the Tranch War, the Pale Throng is still very much active. They have been linked to scores of mutant rebellions across Imperial space, some simply razing an industrial outpost while others have claimed the lives of hundreds of thousands. Notable actions include sparking massive mutant uprisings across the Adrantian and Malfian Sub-Sectors, setting off nuclear weapons on Sepheris Secundus, and the hijacking of the ore barge Borombarst by Rogue Psykers that caused the destruction of Fenksworld's orbital docks. One of the most infamous Pale Throng attacks was on Baraspine, where the the entire planet was nearly destroyed in an attempted daemonic incursion under the direction of the Chaos Cult of Sorcerers known as the Ghol Voluptara, who had concerted the Pale Throng attacks as cover for their own plans. Only a quick intervention by the Adepta Sororitas prevented the Sorcerers' dark plans from passing.
At the centre of any Pale Throng-led uprising are the Shroud Masters, who still remain despite the destruction of their council in the Shroud Wars. Each Shroud Master in turn operates a retinue of Rogue Psykers and veteran or particularly powerful Mutants. This cell infiltrates a world, where the Shroud Master and his minions begin to spread the name of the Pale Throng to the mutant and downtrodden. Any resistance or voices of dissent among the mutants is viciously put down by the Pale Throng to ensure that an uprising takes place under their direction. When their pieces are in place and the mutants organized, the Pale Throng will launch an utterly unsubtle all-out assault across the planet with their full fury. The Mutant hordes and psychic atrocities will seek to slaughter all humans in their path until they are either annihilated or victorious. Due to their poor planning and inexperienced field-commanders, the Pale Throng will eventually succumb to an Imperial counter-attack, by breaking into small cells and warbands at the end of the conflict. But this again proves to be an advantage as they scatter and go into hiding.