I got stuck in a 1 hour delay on the subway with this page open, so here have an Omake:

Uirus eyed the assembly of cultists warily. The past few hours had been… chaotic to say the least, even if that was the namesake of his own allegiance. Chaos was no stranger to infighting but subduing the other cults had been a struggle and a half. Even though the forces of Tzeentch persevered through sheer numbers and loyal execution of careful plans, it was little assurance given the nature of the threat they were facing.

Every single one of the cultists, Tzeentch included, were bound by heavy chains imbued with sorcerous runes, creations of secrecy kept from even the highest of their own ranks. Yet a part of Uirus still held doubt, torn between whether they were needed at all or whether they would even hold. After all, his probing of the things that made up the Malum PDF had shown no sign of the Warp, not even a shadow like those he knew were cast by the creatures of the genestealers. It was even less comfort that they had practically no idea of what the thing could do, outside of its fleshcrafting abilities. Dark lords, they didn't even know the name of this new enemy and were just calling it 'The Thing'.

Given what he had read from the few tattered tomes on the history of the Ghoul Stars, was this what had swallowed the fate of whole legions in this region? Was this what the texts had called the Pale Was-

"Now, Uirus. Let's start the test." said Ahsael, interrupting Uirus from his internal musing. All eyes in the room followed the small warpfire knife in Ahsael's hand as he charged the heat of its flames in a brazier that glowed blue with an ethereal flame. He paused to examine the knife, flipping it to check its other side. "We're going to draw a little bit of everyone's blood."

"What are you going to do to us?" whimpered the governor. His cheeks still bore the bruised marks of when Uirus had grabbed his sniveling wreck of a face and shoved him into the chains. In truth Uirus knew he had no need to be so rough, but after hours of forcing Khronate beastmen into chains and gagging their still frothing mouths, he was in no mood to negotiate with the rest.

"We're going to find out who's the Thing." Ahsael continued while heating the knife in the brazier with one hand, the one kept resting on the flamer hung to his side. "Watching Festil back there gave me the idea that maybe every part of him was a whole. Every little piece is a sentient being, with a built in desire to protect its own life."

Ahsael affixed the now silent governor with a stare, the lit eyes of his meticulously sealed helmet casting a dim light onto his face. "You see, when a man bleeds, it's just tissue." He swept his gaze along the line of cultists, many shirking from eye contact. "But blood from one of you Things won't obey when it's attacked. It'll try to survive. For example, by crawling away from a hot knife."

It makes some sense, thought Uirus. Fighting whatever had corrupted Festil had was like fighting an angry amorphous blob that when split in half with by chainsword, had only turned into two more enemies to fight against. It - for what passed for Festil was certainly no man - had moved and struck with a force that no mere mortal could bring to bear. Riddling its flesh with bullets had only slowed it down, it had taken the flames of promethium to finally kill it for good. At the same time, he felt a sense of unease. The Thing had attempted to use Festil's identity to infiltrate the council as a surviving witness of the tunnel battle, with the excuse of bringing critical information on the enemy. That showed a modicum of intelligence at least. True, their attempts at using the powers of the Warp to thwart it had not yielded any results, forcing them back to more mundane methods. But was it really so simple?

"Uirus. Show them." spoke Ahsael, eyes still on the cultists.

"As you wish, my lord" intoned Uirus somewhat distractedly. A small puff of flames from a jury-rigged sprayer briefly engulfed his arms and the small scalpel he held, cleansing them of any possible impurities. Quickly, he took off his suit gauntlet and in one swift motion, sliced his finger, dripping blood onto a small bowl, and put back on the gauntlet. He handed the bowl to the waiting Ahsael.

"Now step back." commanded Ahsael. Uirus complied, though hidden behind the safety of his sealed helmet, his eyes narrowed at his superior. Did his brother not trust him like the cultists below them?

There was silence among the crowd in the audience chamber, only punctuated by a small hiss as the heated knife held by Ahsael touched the blood sample. Nothing else happened.

"As I expected." muttered Ahsael. "You're good. Clean the scalpel, I'll demonstrate what I already know. Bring me the blood from the rest of them once you're done."

Ahsael repeated the actions Uirus took, though Uirus felt that his motions were just a slight rushed. All the while, Uirus cleaned his scalpel gently with warp flame so as to not heat it too much and trigger whatever the thing was, yet remove any potential residue. As he finished cleaning off the last bit of blade, he could hear whispers from the crowd upon hearing the same dry hiss of heated knife vaporizing his brother's blood.

"This is a trick" growled a heavily-built cultists in the second row. "It's always the case with you Tzeetchians." He recognized the muscled man as a devotee of Khrone, but one that was cognizant enough to be appointed as a delegate to the council. Even if for Khronates that meant only resorting to crude insults and demanding duels instead of flying into a rage at the slightest provocation. Not that it had prevented Uirus from breaking every single limb of the man while stuffing him inside the chains.

"We'll see." said Ahsael, clearly holding back his indignity for a more appropriate time. "Let's try Kalak."

He moved towards the shattered corpse of Kalak the Bronze-Blood, lying entombed inside a sealed coffin barely large enough for his body, multiple runes carved on its surface to keep its contents secure. Or rather in this case, to keep what was inside from escaping. It had taken scores of cultists alongside the powers of all the Tzeentch marines to take down the uncompliant beastman. Even then, it had cost them their Rubric Marine, his armor somehow pried open by the incomprehensibly superhuman grip of Kalak's claws, the dust inside scattered to the wind by multiple grenades used slow their opponent down. Given that Uirus had previously fought the beastman to submission with what was effectively a hand tied behind his back, there was no doubt in his mind that the corpse was no Kalak but a Thing wearing his skin.

So when the warpfire knife touched the sample of Kalak's blood with an uneventful hiss, Urius found himself reciting incantations to prevent his shock from being shown to the mortals. Even Ahsael seemed to pause to doubt himself.

"So Kalak was clean, huh?" the governor chuckled, raising a thin eyebrow with amusement with no regard to the danger he was literally chained in. A suspicious reaction given his previous sentiments, Uirus noted. Yet the governor continued to incriminate himself. "The irony of it being the mutant that was found to be clean, a loyal follower to the cause. A loyal follower that you murdered."

Uirus' swept his flamer to target the governor and grunted a wordless threat, waiting for the order to purge the wretched and potentially corrupted man from existence. But even in face of the blatant insult, Ahsael remained silent in thought. He was about to break the tension to ask for the soul of the impudent governor when Ahsael cut through the silence -

"Hand me the next one."

Fair enough, Uirus reconsidered. There's more important things to do. We can dish out the discipline another time. He wordlessly sliced the thumb of a cultist in the line, this one a wincing Tzeentchian worshipper which he recognized had helped with the subjugation of the Khornates, and had not doubted the subsequent directive to lock themselves within the chains. He handed the bowl of blood to a waiting Ahsael and moved on to quickly clean the knife and fill another bowl with the blood of another in line without needing further instructions. They had a lot of testing to do and the abominations within the Malum PDF crawled ever deeper into Janus by the second. All of this work, just to find a single missing body they had kidnapped from the PDF a week ago.

"This is pure grox shit." a chained Lord Janiel spat, somewhat uncharacteristically for such a typically well-spoken worshiper of Slaanesh. Uirus filed yet another suspicion to a growing list he kept in his head. "This won't prove a damn thing." The man looked strange without the canopy of fashionable silks he usually clad himself in. Though part of that definitely had to do with the black eye he sported from being jumped by Tzeentchians sent to arrest him while he slept.

"I thought you would feel that way, Janiel." Retorted Ahsael with a tone of frustration. The stresses of dealing with the enemy hiding within their own ranks was getting to him, Uirus realized. This was not a tactic the Thousand Sons or really any adept of Chaos often faced, with it usually being the other way around. "You were the only one who could have gotten to the ritual circle. We'll do you last." he declared, maintaining eye contact with the defiant Slaaneshi as he plunged the flaming knife into the blood held in the bowl -

Only for the blood to explode.

"FRAK!" cursed Ahsael as the shards of the bowl further shattered on the ground. The detonation had flung droplets of blood onto the rockrete which now raced past his feet to a dark corner of the room as if possessed by some daemon. Yet Uirus, like before, could not feel even a slightest hint of the Warp from this impossible display of biology.

Wait a second. This blood was…

He snapped his head back to the previous Tzeetch cultist. He was not the only one in the room to do so.

The cultist was trembling. The tremors spread to every single visible surface of his body, as if every part of his skin was jumping on its own. The vibrations resonated along the entire line of heavy chains the cultist - no, the Thing was bound to, rattling every man who was unfortunate to be bound with the same chain. The room was now filled with screams from said unfortunates, any stoicism they had left completely broken from being shackled to the biological horror had been terrorizing their ranks for the past few days. The Thing stretched its mouth impossibly wide, adding its inhuman screams to the cacophony.

No, Uirus realized. It's not screaming. It's laughing.

He brought the flamer to bear on the abomination but before he could press the trigger, something whipped out of the Thing's mouth into the muzzle of the device. A long tendril snaked into the bore of the flamer. When he depressed the trigger, he could feel resistance from it jamming the internals, clogging the valves, preventing the promethium from flowing to the igniter. With a jerk of his arm that required more power than he expected, Uirus ripped the tendril from the flamer…

Only to have the tip of tendril above his gauntlet's grip detach and zip back inside.

This is ridiculous. Uirus complained to himself inside his head. How the frak is it doing any of this without any kind of sorcery? In a microsecond of thought aided by the time-slowing effects of his astarte hormones, he created a backup plan. With a single motion, he hurled the bricked flamer at the Thing while drawing his bolt pistol…

Only for the Thing to rocket up out of the enchanted chains like a missile, contorting its flesh at a speed that even the most skilled of the flesh-warping Haemonculi would have trouble doing. It was fully clear in the air as the fuel tank of the flamer detonated with a shot from his pistol. Ignoring the screaming of the insignificant mortals swept up in the firestorm, he rewinded in his mind the previous few seconds to understand how the enchantments had failed to prevent the Thing from escaping.

The scene played out as the Thing latched on the ceiling in real time. There. The glowing runes looked… off in the recording. Uirus' eye widened as he watched the replay show in slow motion the runes peeling away to fuse with the body of the abomination, even as he fired a bolt at the Thing on the ceiling. Those aren't runes, he realized. They're skins of luminescent flesh mimicking the runes. Sure enough, a brief glance at the chains showed that a section of it had been etched out and deactivated, as if by some acid.

The bolt slammed into the Thing, penetrating its flesh like a spear before detonating… behind it in the ceiling? No, not quite, Uirus corrected, it warped its flesh around the trajectory of the bolt like a sleeve to avoid triggering the detonator cap. Clearly they had vastly underestimated the extent of its abilities. It leapt from the ceiling to the ground, no longer the shape of a man but a bipedal mass of pale green flesh with tentacles jutting out of it. Uirus sensed a surge in power behind him, but he dared not take his eyes off the Thing to verify what he suspected was going on.

Pin it down, came the voice of Ahsael, projected into his mind. Uirus allowed himself to feel a tinge of pride in his risky but calculated guess. He aimed his bolt pistol, acting as if he had not yet comprehended how the Thing had evaded its munitions, and fired round after round into it. Each round passed through the Thing's mass, slamming into the rockrete and the occasional cultist behind it, but he did not care. Killing the Thing was definitely more important, what matter was it they created several corpses as a by-product?

NOW, boomed the psychic command of Ahsael. Uirus stepped aside as a blast of lightning arced through the air and into the abomination that stole the face of their cultist. The sound coming from the Thing which Uirus belatedly realized had been laughing this entire time transitioned smoothly into an bone-chilling screech. For several agonizing seconds, Ahsael maintained the lightning, cutting off only when the spell he wove ran dry.

Crumpled on the ground was the charred, misshapen mass of the horror that had terrorized the ranks of the cult. It sizzled and popped with the excess heat that had cooked it into submission. Yet even in its defeated form, Uirus could see one of its less blackened portions of flesh slough off and attempt to inch away to lick its wounds. He strode up to this detached lump of flesh with vengeance and stomped his boot into it, crushing it underfoot.

It was an impulsive decision he quickly came to regret.

The mass of flesh popped, releasing a flood of mite-sized creatures scattering in every direction. Some ran for the shadows, some for the cracks in the rockrete, and others disappeared beneath the feet of the cultists, tied down by their chains and unable to do anything but shriek in terror.

Uirus looked around in defeat, and met the gaze of Ahsael, still breathing heavily from the sorcery that had extracted a toll from him. Despite all that had happened, Ahsael had nothing to say. As his gaze wandered over the masses of the cultists who were each in various stages of grief, he found the tired eyes of Lord Janiel.

"I know you gentlemen have been through a lot." The Slaaneshi cult leader offered diplomatically, trying to ignore the muffled screaming and squirming of two Khornate cultists he had been tied in between. "But when you find the time... I'd rather not spend the rest of this session TIED TO THIS FRAKKING COUCH!"

They did not untie him.