He smelled it long before he reached the lab. The sweet smell of rotting meat, but not quite like the real thing, tinged with musky pungency and the sting of pheromones not intended for his nostrils. The stench only led him to rush even more, driving him to transform into Silberblut pre-emptively.
His fears were only further affirmed by the sounds of commotion as he sprinted through subterranean corridors lit only by old, flickering lamps. Three layers of black-iron doors had separated the lab from the corridor, but now, there was just a tunnel of torn-up stone. The doors were embedded into a wall inside the lab one atop the other, having been blasted from their hinges and smashed into a monolith of abused metal by immense concussive force.
The unmistakable voice of Tsetse echoed from within: "...possess something that was stolen from me. Return it, and I will let you live. Lucky you."
"I- I don't know what you mean, truly!" Cornelius insisted unconvincingly. His eyes jumped to Casus when he passed through the door. Scanning the situation before him, the first thing that hit him was the state of the lab; surprisingly, not wrecked. There was damage, yes, and quite a few pieces of equipment had been destroyed, but it looked plausibly collateral. As for Tsetse, he had cornered the swarthy grafter, who was keeping the flyman at bay thanks to a quarter-circle of blood drawn on the floor. Using it as a catalyst, Cornelius generated an immensely potent barrier. Its weakness against Tsetse's Kinetic attacks was offset by the fact it lashed back at him, as evidenced by the still-smoking Seven Spokes insignias that had been mercilessly branded onto Tsetse's body.
Indeed, Cornelius was truly exceptional when it came to purely defensive thaumaturgy, and not just in terms of barriers. His wards, too, were downright excessively thick, interlayered and compound, unreasonably complex for his distinct lack of combative tendency. That was, after all, why he was so defensive; he utterly lacked the nerve to even fight back. He was the one worm who would sooner grow a spiked shell on the spot rather than turn and strike back. No matter his talent, however, Cornelius couldn't hold that barrier up for long. He knew this, and Tsetse knew this, and Casus knew that Tsetse knew.
For that reason, a flash of hope lit up in the grafter's eyes when he saw Casus, and he immediately called out to him: "Ah, thank Zavesh you're here! A-as you can see there has been a bit of a misunderstanding, please explain to my friend here that I don't know anything about his arm!"
As mentioned earlier, Cornelius didn't have the nerve to fight. For that same reason, he also couldn't lie to save his skin.
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Tsetse turned to meet his gaze, and, despite the flyman's stoic visage, the noise he emitted was very much that of a chuckle.
"Ah. Lucky me. Silberblut," he said, sounding genuinely glad to see Casus. "I must thank you, the data from our fight led to quite a few improvements to my morph, as you can see, and as you will soon feel for yourself. Worry not; I can feel your confusion. If you survive a head-on strike from me, I will freely divulge the nature of my existence."
While he spoke, Casus used these precious moments to inspect Tsetse's altered form. The sonic emitter bulges over his ankles had been joined by another pair just above the knees, and both pairs were now more elegantly melded into the curvature of his plating, with eye-like slits in the chitin. His arms were completely different from before. The right still missed its lower half, with a crude-looking machine prosthetic in its place, cables winding up the limb to a compact power unit embedded in his back. As for his left arm, it had bulked up as if to compensate, individual plates now spread apart by bulging muscle. Any trace of sonic emitters was gone from the limb.
"I presume you know where it is. Please, disappoint me by disclosing its location without a fight."
Casus knew, of course. The arm was beneath them, its container one among dozens within a mechanized storage system. The access panel was in fact right behind Cornelius.
Rather than respond verbally, he simply dropped into his fighting stance. Having been transformed for a short time already, he felt mostly confident that he could pull out a coupler charge right on the spot. It wasn't a good idea as he risked backlash, and even if successful it would cut into his stamina quite severely. However, given the state of those doors and Tsetse's confidence, Casus wagered it was his best bet.
Tsetse raised his left arm to waist height, hand clenched into a downright weird fist. The exposed muscles of his torso flexed, and plates snapped out of place to reveal an array of three large and nine small sonic emitter lenses. Their placement was awkward, spaced out widely near the sides to make space for Tsetse's powerful core musculature. That explained the thick plates on his sides - their purpose was to protect the emitters.
Tsetse followed with a short punch, like something one would use in tight quarters into an opponent's stomach. Casus, reading it as the trigger gesture for the greater emitter cluster, opened the Second Eye. Just as a wall of force came bearing down on him he devoured it, skidding back just a bit. However, another blast of force came just as the Second Eye's window of effectiveness petered out. Viciously focused, it wasn't just enough to throw him against the wall - it embedded him into the brickwork and continued on through him, carving a hole into the stone. The Silberblut armor's internal structure as a relic of the highest order was the only thing that prevented it from turning his insides into mush. Nothing could severely harm him until the armor's durability was depleted, no matter how focused the attack was. Unfortunately, after weathering that, there really wasn't much durability left. He wagered he could take maybe one more hit like that, and perhaps one or two regular strikes after that before he was forced out of his transformation.