Papyrus crept quietly past his sleeping brother, careful not to wake him. He didn't want him to freak out when he figured out where he was going.
   The taller skeleton gradually made his way to the door, opening it slowly, praying that it wouldn't make any sound as he slipped outside, closing the door silently behind him.
As soon as he was out of the house, he let out a sigh. "I probably shouldn't be doing this... Sans is gonna lose it when he finds out..."
He stared out into the white mist shrouding the transition between Snowdin and Waterfall, reconsidering his decision to leave.
You don't know what's out there, Papyrus.
The skeleton flinched at the sound of his brother's voice echoing through his skull. It was so vivid that, for a moment, he wondered if Sans had actually said it. But he quickly realized that it wasn't the case, as he was nowhere nearby.
   He simply shrugged it off, and started walking into the mist, determined to find his best friend.
Turn back bro! It's not safe out there! Come back home!
   Papyrus froze as his brother's voice reverberated through his mind once more. He chuckled, an uncertain grin on his face. "Am I really going to go through with this??"
He thought for a second before shaking his head, continuing in his path regardless of everything that seemed to be screaming at him to turn around and go home.
"No. She's my friend. It's my job to worry about her well-being!"
   The skeleton walked with purpose, his head held high. He'd made up his mind. He was going to find her, no matter how much the vivid echo of his brother's voice shouted at him to let it go.
**************************************************

Undyne took off running, cursing frantically under her breath while tightly clutching her forearm. Blood welled up from the fresh wound, trickling down her arm from underneath her hand.
She'd never been so unbelievably freaked out in all her life. Not much could scare her, but this was an exception. For once, she was unsure of what she was doing. Unsure of what was yet to come.
All she knew was that she had to get away. Away from the rest of the Guard. Away from any innocent civilians that might be nearby. Away from everyone.
The fish monster eventually came across a dark cavern. She paused by the entrance, panting heavily. She contemplated her options for a moment, but a sharp sting of pain in her arm made the decision for her.
"Guess this'll have to do..." she mumbled, sprinting into the pitch-black darkness of the cave and disappearing from sight.
*************************************************

It wasn't long before the skeleton was in the heart of Waterfall. He glanced around, finding that there was nobody in sight.
"Where is she?" he murmured to himself, continuing forth along the path leading all the way through Waterfall.
While he was indeed focused on finding his friend, he couldn't help but anxiously think about where she could be.
"Hopefully she's not hurt..." Papyrus whispered to himself, his optimism slowly beginning to give way to a crippling sense of fear.
*************************************************

Undyne finally reached a dead end.
The fish monster glanced around, wincing as she felt another stab of pain through her arm. "I-I think this is f-far enough..."
She sat down with her back against the cavern wall, taking a moment to wipe the sweat off her forehead and take a look at the massive bite wound on her arm. Her eyes went wide as she realized that the sickness was already beginning to spread.
The blood that was still gushing from the gashes had turned slightly greenish in color, beginning to mimic the color of her magic (which was oftentimes the first warning sign of a monster becoming infected), and she could see the veins in her arm bulging out slightly (which wasn't a good sign either).
"S-Son of a b-bitch..."
She had no choice. If she were to avoid being infected completely, she'd have to figure out a way to cut off circulation to her arm.
Not taking a moment longer to think, she grabbed ahold of the bottom of her tank top using one hand. She pulled the fabric up level to her mouth, biting onto it and yanking back on it with her hand.
The shirt tore without a problem, and she frantically began to wind the strip of black fabric tightly around the spot just above her elbow. She made sure it was as tight as it could be, yet she was still afraid that it wouldn't be enough.
She sighed, closing her one good eye and resting the back of her head against the cave wall behind her while still clutching her arm with a vice-like grip. Her ear-fins drooped slightly as she tried to relax, but with no success.
She opened her right eye and glanced back at her arm, a chill running up her spine at the sight of the massive bite marks that gouged into her flesh.
"This can't be h-happening to me..."
*************************************************

Papyrus eventually stumbled upon the mouth of a cave. At first, he didn't think much of it. That is, until he noticed a few drops of blood, oddly tainted with a dark (almost black) green color.
"That's weird..." he mumbled, scratching his head with his gloved hand in confusion. The confusion soon melted away into panic as a different thought came into his mind. "What if there's an injured monster in there??"
He closed his eyes for a moment, a bright neon-orange light shining in the sockets when he reopened them. He then peered into the darkness of the cavern, the structure illuminated by the glow of his eyes.
He noticed a couple more specks of the green tainted blood not far from the original spot he'd discovered it, feeding into his concerns. Without wasting any time, the tall, lanky skeleton began to follow the thin trail of blood, hoping to find its source at the end of the path.
************************************************

Undyne was alerted to the approach of another monster by the clatter of boots against the rock floor of the cave. Her right eye snapped open, and her ear-fins perked up a bit as she searched for the one who was fast approaching, ready to chase them off if she needed to. The last thing she needed was to be the cause of another innocent monster becoming infected.
It wasn't long before she heard a voice that normally would have brightened up her day, but now gave her a terrible sense of dread.
"-Is there anybody there?"
The fish monster didn't respond. She was too terrified of the situation to say something anyways.
"-Hello?"
The heavy clomping of boots came closer.
"-If you're here, please say something!"
   Clomp, clomp, clomp...
Undyne held her breath.
More clomping...
Then, silence.

Undyne stared into the darkness, her stomach lurching as she instantly recognized the tall, lanky figure of Papyrus, illuminated by an eerie, neon-orange glow.
"U-Undyne?"
The fish monster's face grew pale. "P-P-Papyrus?"
The skeleton rushed forward, his eyes alight with concern. "Are you hurt?! What's going on?! Why aren't you with the rest of the Guard?!"
He reached out to rest his hand on her shoulder, in which she instinctively slapped it away, her teeth bared. "No."
Papyrus seemed taken aback, flinching away from the fish monster at her unexpected response. But, in that short amount of time she moved her hand off her arm, the infection slowly began to work its way further up her arm. She winced and gritted her teeth, instantly gripping her arm in an attempt to prevent further spread of the infection.
She inhaled sharply before looking up at her friend, a fearful gleam in her eye. "Look, y-you can't be h-here. It's n-not safe..."
Papyrus narrowed his eyes, kneeling down next to her. "What do you mean 'it's not safe'?"
"Papyrus. You n-need to go. You c-can't be here. N-Not right now," the fish monster repeated, desperation trickling into her voice.
"What's the problem?" He finally noticed that she was clutching her arm, the green-tainted blood gushing from the wound smudged by her hand. "You're hurt!"
Undyne flinched, her fingernails digging into her own flesh as she gripped her arm even tighter. "I-It's fine! I'm fine! J-Just get outta here, d-dammit!"
"Stop! You're hurting yourself!" Papyrus exclaimed, rushing forward and grabbing onto her arm, trying to pull her hand away from it.
"Papyrus! Jesus Christ! Just stop!" Without thinking, the fish monster kicked out with her legs, throwing the skeleton backwards.
In that moment, she loosened her grip slightly, allowing the disease an opportunity to spread further through her. She grunted softly, doubling over and clawing at her wrist. Papyrus took a step back, determination sparking in his eyes.
   "I won't stop! You're injured! And it is my job, as your friend, to help you in times of need!" He advanced towards her slowly, his gaze softening. "So please... Just stop trying to push me away..."
   She glanced up at him, a pained expression appearing on her face for a brief instant. "Trust me... I'm telling you to get the hell outta here for a reason..."
   Papyrus fell silent, staring down at his boots to avoid looking at his friend. "I'm not leaving."
   Undyne's gaze softened. "Look. I get it. You're a very kind and caring monster, Papyrus. But you c-can't be here..."
   "I don't like seeing others in pain..." Papyrus murmured, turning his head away from the fish monster.
   "I-I'm in a lot more pain now with you just s-standing there... Nothing h-hurts more than seeing you here, and being forced to p-push you away..." Undyne responded, her voice somewhat strained as she tried to keep her cool.
Papyrus squeezed his eyes shut at her words, tears pricking the corners of his eye sockets. Undyne noticed, and reluctantly beckoned him to come over, which he gladly did.
   The fish monster inhaled sharply between her jagged fangs as she shifted position ever so slightly, half-leaning into Papyrus. The skeleton looked down at her, but said nothing.
   She slowly loosened her grip on her arm, testing to see how fast the infection could spread in just a short amount of time. She winced as it suddenly surged up her arm, reaching past her elbow, despite the tight knot tied around the area in her desperate attempt to halt its progression.
   She cursed softly under her breath, pressing her hand against the wound once more. Papyrus looked over, his eyes dark with concern. "You okay?"
"I-I don't even know, Pap..." she murmured, staring down at her arm and watching as the area where her veins were bulging out began to turn to a dark green shade. She shuddered at the sight, yet she was unable to pull her gaze away.
Her gaze trailed down to her hand, her heart nearly skipping a beat as she noticed that where she'd once had normal fingernails, she now had animalistic claws. She went into panic at the sight, her eyes going wide and her breathing beginning to escalate.
Papyrus looked to her in concern. "You really don't look too good... And you're not acting quite like yourself... What's going on?"
Undyne didn't respond; she was much too freaked out to say anything at the moment anyway. She gritted her teeth as pain shot all the way up her arm, extending slightly past her shoulder.
Papyrus twitched, clearly picking up on his friend's agony. Undyne glanced up at him, her gaze weary. "Y-You really shouldn't be here..."
"But I am," the skeleton responded, putting his arms around her in a hug.
Undyne allowed him to comfort her for a moment before she abruptly pushed him away, just as she felt another sharp wave of pain in her arm, the sensation spreading further across her body and slowly making its way to its destination: her soul.
"I've let you s-stay this long, but you n-need to leave. Now. A-And that's an order, Papyrus." She stood up, stumbling back a few paces to put some distance between the two of them.
He narrowed his eyes, crossing his arms in defiance. "Then I guess I'm being insubordinate when I say that I'm not going anywhere."
Her grimace twisted into a snarl at his response. "I'm d-doing this to protect you, d-dammit! Why c-can't you see that?!"
"I can protect myself! I'm not a child!" Papyrus said defensively, standing up to face the fish monster.
Undyne narrowed her eyes, but before she could say anything, an intense jolt of pain in her chest caused her to double over, crying out in agony as she dug her claws into the center of her chest, right where her soul was under the surface.
Papyrus was caught off-guard by her sudden actions, yet he still took a cautious step forward, ready to catch his friend if she fell.
She glared up at him, a mixture of pain, fear, and slight anger gleaming in her uncovered right eye. She was still tightly grasping her arm, despite knowing that it was already too late to stop the infection's spread.
"I-I don't need your h-help. I need you to g-go away," Undyne hissed, the tone of her voice coming out much harsher than she intended.
Papyrus reluctantly took a step back, a disheartened look on his face. Seeing him like that made Undyne feel guilty, but she knew that she'd feel even worse if the skeleton became infected because of her.
She opened her mouth to say more, but the only sound that came out was another cry of agony as pain seared through her, the source of it being where her soul was.
Papyrus's gaze snapped to the fish monster, his eyes wide. Undyne shot him a look of desperation, wordlessly pleading for him to leave. She knew that she couldn't hold it back forever, no matter how hard she tried.
Dammit... Why are you so goddamn caring, Papyrus?
"L-Look, I have no idea h-how this is gonna end, but it's not gonna be p-pretty. Please... Just g-get out while you still c-can..." she murmured, her voice straining from the effort of trying to stay calm.
Papyrus glanced down the tunnel he'd come from, taking a moment to consider it before looking back to Undyne and shaking his head. "I'd rather stay here with you."
Undyne was about to keep insisting, but something stopped her.
Screw it. If he's so damn stubborn, let him get hurt then. He had the chance to run already.
She didn't have any time to think about the strange, beastly voice she'd heard echoing through her mind before she was suddenly overcome by an intense wave of pain, causing her to completely fall to the ground.
   She planted the palms of her hands on the ground, catching herself just before she could hit her head against the stone floor of the cavern. Papyrus took a step towards her, but froze as Undyne suddenly slammed her fist down on the floor, cracking the surface below her hand.
   Papyrus was used to seeing the extent of Undyne's strength; she would pick up huge boulders and chuck them across her yard just to show off, for crying out loud!
   But, for some reason, this seemed different.
   The skeleton didn't have much time to sit and ponder before he was jolted from his thoughts by a scream. He flinched, clearly not expecting the sound to be coming from his friend, who was normally very strong- both physically and mentally.
   His gaze snapped to the fish monster, and he had to slap a hand over his mouth at the sight that greeted him.
   As he stared at Undyne, he quickly realized that her form was somehow changing... A small ridge had formed along the back of her tank top, and her nose and mouth had begun to stretch out into a muzzle.
Within moments, her tank top tore completely down the back, revealing a fin that ran along the entire length of her spine.
   The fish monster snarled softly as she suddenly began to claw at the eyepatch that covered her left eye. She tore it off in a matter of seconds, throwing it down on the ground when she'd taken it off.
   Papyrus took a few steps back, unsettled by what he was seeing. He twitched as her gaze locked onto his. He had never seen what her left eye was like beneath the patch; it was blacked-out, with a golden-color pupil.
   Despite the crazed gleam in the fish monster's eyes, there was still a spark of desperation behind them. "P-Please... G-Get out of h-herre..."
   Papyrus was caught off-guard by how different his friend's voice sounded all of a sudden. While he could still hear her normal voice, there was a distinct growl underlying it.
   Undyne held his gaze for a moment longer before pain tore through her once more, forcing another slightly animalistic scream to escape her. She clawed at the sides of her face once more, this time, in response to a sharp pain along her cheekbones.
   As she scratched, she could feel something jutting out from beneath her skin. She glanced at Papyrus, who was just staring back with a look of pure terror on his face.
   She was about to plead with him once more, but the words caught in her throat and instead came out as another agonized wail as she felt her flesh tearing, just as two large spikes shot up from her cheekbones on both sides of her face.
   Watching Papyrus stumble backward in horror almost brought her to the verge of tears. But she didn't even have time to shed a single tear before the pain struck once more, this time, closer to the top of her ear-fins.
   She dragged her claws down her face as she felt two more spikes rip through her skin, though these were more like actual horns than anything. She could feel blood trickling down the sides of her head, mixing with both her sweat and her tears as it dripped onto the ground.
   Papyrus, now leaning against the cavern wall for support, stared at his friend with wide eyes. "Wh-What's going on?!"
Undyne's head snapped up, her gaze locking into his once more. Papyrus flinched as he realized that the face of his dear friend was now nearly unrecognizable. At this point, both her eyes were completely black, with the exception of her gold-colored irises, which were completely fixated on him.
"G-Get the hell outta herre, Papyrrus..."
Papyrus felt a chill crawl up his spine at the sound of his friend's voice. Not only was the underlying growl from before even more prominent now, but the tone of it had deepened... It didn't even sound like Undyne at all.
   He opened his mouth to argue, but he was interrupted by another loud scream from the fish monster. It took him a moment to realize what was wrong, but he quickly figured it out.
   The structure of her legs was changing now, her feet elongating, tearing apart her boots and jeans in the process. Though he couldn't put it together initially, Papyrus began to realize that the way his friend's physical anatomy was changing almost resembled a dragon of some sort... Not like Sora, but like an actual, mythical dragon.
   While he was pondering this thought, he managed to completely tone out the sounds of Undyne's horrifying transformation, just as it really began to hit her.
   Her form was drawing on a mass it normally didn't have, her body becoming more heavily muscled while also growing much larger than anything Papyrus had ever seen. More spikes tore through her flesh, right where her heels would have been in her regular form, which was now on the back of her legs.
   By the time the skeleton was pulled from his thoughts, Undyne's transformation had come to an end. He stared up at her, his eyes going wide at the sight of the massive beast his friend had become.
   "U-Undyne?!"
   The only response that the now reptilian-looking creature gave was a low snarl as she stepped towards him, lowering herself to the ground and making herself at eye-level with him. A long tail, with large fins at the end of it, flicked out behind her, taking the skeleton by surprise.
   A tail? Since when did she have a tail??
   Papyrus cautiously side-stepped, towards the direction of the exit. The creature's gaze followed, and her snarl seemed to increase in volume as she realized that he was planning to escape.
   He froze, fear creeping into his thoughts. "P-Please... I-It's me, Papyrus... Y-You've got to remember me, r-right?"
The beast suddenly lunged forward, a load roar escaping her as she launched towards the skeleton with wide-open jaws.

Boom! Cliffhanger! XD
How was that? Is that the kind of transformation you were expecting to see first here?
I actually had a lot of fun writing this part. I really haven't had much experience with writing transformations like this- y'know, with flesh and stuff instead of just bones. But it's good to get outta your comfort zone every once in awhile! :D
And for those who were wondering, I do have a drawing that I did of Undyne's beast form.

So yeah. This is what she looks like.
Anyway, I hope you enjoyed! I'm already in the process of writing the next part, so stay tuned!
-Sora