The whole world seemed to freeze around Papyrus as the beast that had once been his friend clamped her jaws down on his arm. The sickening sound of bone crunching broke the silence, and it was followed by a loud cry of pain from Papyrus.
   The massive reptilian-looking creature eventually let go, a wide-toothed grin spreading across her face. Papyrus stumbled back, tightly clutching his forearm. Blood gushed from the wound, seeping through the red fabric of his glove as he struggled to keep pressure on the wound.
The bone had been completely shattered, the shards only being held in place by the strength of his own magic.
   He stared up at the beast, a terrified gleam in his blazing orange eyes. "U-Undyne?! Wh-Why would you..." He trailed off, glancing back down at his arm.
   His eyes went wide as he noticed a strange vein-like pattern becoming visible on his arm, glowing with the same color as his magic. The blood that leaked from the bite marks was slowly becoming tinted with a neon-orange hue, mimicking his magic color.
His breathing escalated as he felt a jolt of sharp pain through his arm, starting where the bite wound was and extending up to his elbow. He winced, gripping his forearm tighter in response to the pain.
   The beast that had once been Undyne stepped back, a twisted smile on her face as she watched the events unfold. She even laid down on her belly, appearing relaxed. She knew what was happening. But she lacked the rational sense of thinking that was needed for her to feel guilty about what she'd done.
   Papyrus glanced at her for a moment, but couldn't hold her gaze. He allowed himself to slump against the cave wall behind him, slowly lowering himself to the ground as he tried desperately to calm down.
   And he almost succeeded with that task... Until he was snapped out of it by a loud cracking sound. His eye sockets flew open as his gaze darted to his hand, which was where the sound seemed to be coming from.
   Part of him instantly regretted looking down, for what he saw only added to his fear. He lifted his hand, his eyes wide with horror as he noticed that the tips of his fingers were growing sharper, pushing against the worn red fabric of his glove. He could only assume that the same thing was happening to his other hand.
His soul pounded frantically as his mind shifted into a full-on panic mode. He was slowly processing everything that had happened; He'd just watched his friend change into one of the beasts that everyone was hiding from, she'd bitten him, and now... His fate was sealed. He couldn't reverse what had been done.
   I should've ran when I had the chance.
He gritted his teeth as pain shot up through his arm once more, the sensation extending all the way up to his shoulder this time. He took a deep breath, trying to force himself to keep calm.
"C-Come on... Deep b-breaths... Quit f-freaking out... B-Being afraid doesn't s-solve anything..."
But no matter how many times he told himself to relax, he couldn't actually make it happen. A fierce paranoia was screaming in the back of his mind, deterring him from focusing on other things.
"Ah, who am I k-kidding... Nothing is going to s-solve this..." He stared down at his hand, watching in cold terror as the fabric of his gloves tore, revealing the sharp, dagger-like claws that had formed on the tips of his fingers.
   "I-I made a stupid mistake..." He could feel the pain spreading throughout his body, making it even harder to think. "A-And now, I have to s-suffer the consequences..."
   He lurched forward with a cry of anguish as the sickness began to have a stronger effect on him. He could feel his spine beginning to distend, snapping and twisting as it reset itself.
   He drew in a few shaky, uneven breaths before he kept talking. "I-I deserve th-this... I-I should have l-listened when you gave me or-orders, Undyne..."
   The neon-orange inferno in the skeleton's eyes became swirled with a golden hue, giving off an eerie yellow-orange glow that illuminated his shuddering form in the darkness of the caverns.
   The changes were beginning to work their way down into the lower half of his body, causing him to let out an agonized scream as his feet began to splinter and shift, elongating as the tips of his toes grew into sharp claws, which ripped through his boots as though they were nothing.
   In a matter of moments, his legs had completely reformed as digitigrade, and his boots were completely destroyed. His newly-formed claws left deep gouges in the rocky floor beneath him as he struggled to keep himself upright.
   "U-Undyne..." His skull began to change as well, his nose and mouth stretching out into a muzzle as horns emerged along the back of his head. "S-SansSSS..." His voice suddenly became deeper, and edged with an animalistic growl. "I-I'M SSORRY..."
   And that was when the disease finally took its hold, causing all the changes to happen at a much quicker rate.
   Massive spikes suddenly burst out from the back of his t-shirt, mainly down the length of his spine and across his shoulder blades, while several other spikes formed on his elbow joints and where his heels once were.
His ribcage expanded, stretching out the fabric of his shirt and tearing it, eventually reducing it to mere shreds as his form began to grow larger, his bones distending and thickening all at once.
His voice was becoming deeper and increasingly animalistic with each gut-wrenching shriek that escaped him, adding to the horrific nature of the transformation.
It seemed like ages before it finally stopped. Even still, the tall, lanky skeleton remained lying down on the ground, shakily trying to catch his breath and calm himself down.
And for awhile, all was silent, with the exception of Papyrus's rumbling breaths and the occasional sound of Undyne's tail fin brushing against the cave floor.
...That is, until a monster's voice echoed through the caverns.
"...Captain Undyne? You doing alright in there?"
Papyrus leapt to his feet at the noise, a distinct orange glow becoming visible within his ribcage and at the back of his maw. Undyne stood up too, but not with the same urgency that Papyrus did.
The skeletal beast had only one thing on his mind; bloodlust.
Musst... attack... Need... to shhed blood...
At the back of his consciousness, a small sliver of the monster he used to be protested.
No! I'm in the Royal Guard! I would NEVER hurt another monster!
But his body wouldn't do as he wanted it to do. He could only stand by and watch through the eyes of the beast as he charged towards the sound of the voice, ready to lash out at its source...

...And continue to spread this horrible disease that his mind had been completely taken over by.

Woo! Finally got this part done!
Sorry this took so long guys! I was really focused on updating my regular transformation oneshots, so I didn't have much inspiration to work on this...
Anyway! Hope this chapter was worth the wait! I'll try not to keep you waiting as long for the next one, but no promises. :/
Until then, stay determined!
-Sora