Ok, this one is HorrorTale and was requested by UndertaleWeirdo89
I had to use that as the title. XD No regrets.
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   Sans watched from the shadows as a human walked out from the Ruins. He chuckled quietly to himself as he followed closely behind them, his crazed red eye fixated on the meek little child.
   He touched a finger to the small drill attached to his hand, making sure it still worked properly. Perfect. The prank was surely twisted, but it was so fun to see the looks of pure terror and agony in unsuspecting humans' faces.
   The meek human looked around anxiously, jumping at every little sound. Sans giggled softly in a maniacal way. This was going to be so much fun.
   "Human. Don't you know how to greet a new pal? Turn around and shake my hand," Sans growled softly into the human's ear.
   She flinched as she spun around to find herself face-to-face with Sans. She screamed at the sight of the clearly-insane skeleton.
   Sans smirked as he extended his hand out to the child, a devious gleam in his crazed left eye. He barely held back a chuckle as the human was stupid enough to actually take his hand.
   Sans held the child's hand in what was almost like a death grip as the drill on his skeletal hand dug into the palm of the terrified child's hand.
   The child screamed as the drill activated, tears streaming down her face as she fought to get away from the crazed skeleton.
   "What? You want me to let go?"
   The child nodded, desperately trying to escape Sans's death grip. Blood splattered all over as the drill tore into the child's flesh.
   "Heh. Suit yourself," Sans let go, causing the child to stumble back and fall to the ground.
   The meek little human stared up at Sans with a terrified gleam in her eyes. Deep within Sans's crazed mind, he knew that what he was doing was sick... and twisted...
   Something about this scene seemed... cruel.
   Sans shook his head violently to get the thoughts to go away. No. He was not sane. The good side of himself was no longer there. Even if there was something remaining of his old self, it would probably be no more than a mere shred of his past.
   Fight it... Stay insane... The old you is gone... Forget about that old version of yourself...
   Sans's red eye was slowly regaining its original cyan color. He held a hand over the now cyan and red eye as he teleported back to his house in Snowdin.
   Once he got to the house, he collapsed on the couch. Papyrus was in the kitchen when his brother got to the house.
   "Are you okay, brother? You seem a little out of it today..." Papyrus commented.
   Sans took a moment to catch his breath before turning to face his brother. Thankfully, his eye had returned to its normal red color before his brother had seen him.
   "I'm fine Pap. I just saw a human," Sans murmured, his voice somewhat raspier than usual.
   Papyrus tilted his head to the side, but didn't ask questions. Then, he really considered what his brother had just said. He saw a human... and he didn't do anything to capture it?!
"Why didn't you try to capture the human?!"
Sans flinched. Dammit. He'd known this was coming.
He sighed. "Well... this might be hard to believe..."
"Go on," Papyrus pressed.
"That human somehow reminded me of who I used to be. Back when I wasn't insane. I... I couldn't stay there..." Sans said hesitantly.
Papyrus looked shocked at his brother's confession. "What do you mean, 'before you were insane'? Haven't you always been this way?"
Sans looked away. "No. Believe it or not, I wasn't always so sick and twisted like I am now. I used to be... a friendly guy. Then... Insanity struck... I completely lost who I truly was..."
Papyrus fell silent as tears formed in the corners of Sans's eyes. He couldn't tell if he was imagining it, but he could have sworn that he'd seen a hint of cyan in his brother's red left eye.
Sans growled in frustration as he held a hand over his eye once more. "Forget about it. Go to Undyne's house. Don't you have training today?"
"You mean my lesson about what to do if I spot a human? Yes," Papyrus replied.
"Cool. I'll meet ya there later," Sans said as he teleported back to the forest.
It took him awhile to find the human, but he'd finally found her. There she was, about to attempt the first of Papyrus's puzzles.
Something seemed to be screaming at Sans to help the poor child. He had unknowingly rushed forward and stopped the human from walking straight into the hidden bear trap at her feet.
He shoved her aside and dropped a small stone onto the trigger for the trap. The trap snapped upwards, making a loud clang as the metal smashed against the cold stone.
The child waved thankfully at him as she passed by, making her way to the next puzzle. Sans stopped himself, insanity taking back control just before he could do anything else.
He dug his fingertips into his own skull, the slightly sharpened tips from years of his insanity leaving huge gouges as he dragged his hands down his face.
"What is wrong with me?! I just helped that human with the puzzle!" Sans hissed, shuddering as he felt his own blood trickle from the huge scratch marks down his face.
"What are you doing?!" a voice screamed at Sans, causing him to turn around.
Behind him, was none-other-than Queen Undyne. To make things worse, his brother was there as well.
"Uh, I can explain-"
"No. Don't try to explain anything. Papyrus already did that for you," Undyne growled.
Papyrus looked away guiltily. Not far away, the human child had turned around and was heading back towards the trio of monsters.
"So, tell me about this 'fight to stay insane' that you're going through. What's this talk about the human somehow bringing back the sane side of you?" Undyne pressed.
Sans pressed the palms of his hands against the sides of his skull. Undyne bared her teeth as she strode forward and grabbed Sans firmly by his arm, yanking him closer to her.
"I'm not sane... I'm not sane... I'm not sane... I'm... not... S A N E . . ." Sans mumbled, his voice escalating in volume with each word he spoke.
His red left eye was once again tainted by the bright cyan color that signified that his old self was trying to regain control.
"I ' M N O T S A N E . . ." Sans screamed, his voice edged with an obvious echo.
The skeleton looked over at the child, who had snuck back to see what was going on. The concern and fear in her eyes... It made his soul ache.
That was when he realized that the pain in his chest wasn't just in his head. It was real.
Panic struck him like a bullet to the chest. For the first time in a really long while, Sans was scared.
His breathing escalated as the pain and stress built. He could hear Undyne shouting at him, but he couldn't make out what she was saying. The cyan in his eye started to fade away as he lost himself to insanity once more.
All that remained in his left eye socket was a blood red abyss. Sans could feel the pain from his chest beginning to spread throughout the rest of his body, making it incredibly difficult to remain standing.
Papyrus dashed forward and caught Sans just before his skull hit the ground. The smaller skeleton was breathing heavily as he tightly gripped his brother's arms. The taller skeleton winced as he felt Sans's sharpened fingertips dig into his forearm.
   Sweat glistened on Sans's forehead as he tried desperately to keep himself sane. He was so tired of this sickening torture that he inflicted on others. He just wanted it to end.
Suddenly, Sans felt a jolt of sharp pain through his spine, causing him to cry out in pain. He could feel his spine begin to snap and twist as it re-set itself. His dark-blue jacket tore down the back, revealing the large spikes that had begun to form along his vertebrae.
This was it. He was finally getting to feel the same pain and torture that he had caused so many others. Both humans and monsters alike. This was almost like their revenge. Getting to see him suffer for once was surely satisfying to all of his victims.
Sans howled in agony as another surge of pain was sent through his body. His voice was becoming more and more animalistic with each passing moment. His fingertips dug deeper into his brother's arms, the suffering skeleton completely unaware that his already-sharpened fingertips were rapidly becoming claws.
   Sans's already slightly-sharpened teeth started growing into long curved fangs. He became aware of an unusual tingling sensation in the lower portion of his spine, which if he'd had the strength to look back, he would have seen a tail beginning to grow out behind him.
   "Sans, please... You have to fight it... You must stay sane!" the human pleaded, stepping out into the open and striding bravely towards Sans.
   Sans tried to push her away with a clawed hand. "K-Kid, please... I deserve th-this..."
   "She's right, brother. You must stay sane! You can't fall under this insanity once more! Remember who you used to be!" Papyrus encouraged.
   There was a flicker of cyan colored light in Sans's blood red eye for one brief moment, but it was soon gone as Undyne began to speak.
   "No! Don't listen to them! They are weak! They won't survive in this world! Stay insane!" Undyne shouted.
   Sans let out a scream as he threw his head back, his left eye giving off an eerie red flame. His face was beginning to elongate and several horns were beginning to emerge along the back of his skull. There was a loud crack as his face split from his nasal cavity to his browline, causing Papyrus to flinch.
Undyne stepped back, her smile faltering as she realized that Sans's bones had begun to grow longer and thicker, his body drawing on a mass that it didn't usually have.
His voice was completely unrecognizable. It had deepened into raw, feral screams. After what seemed like ages, the transformation ended.
Undyne stared up at the huge beast that Sans had become, the feral gleam in his blood red eye reminding her of something that she'd been faced with before.
She could feel her sanity returning to her, reminding her that she was once the head of the Royal Guard. Her job was to protect the defenseless civilians that inhabited the Underground.
She had faced Sans while he was like this before. She still had the scars to prove it. Undyne felt the last of her insanity fade away as she truly remembered who she was.
Now, she had to remind Sans of who he was. Papyrus had already caught on with Undyne's plan, calling out to his deranged brother in hopes of snapping him out of his feral state.
"Sans! It's me, your brother! You have to snap out of it!"
   The beast roared, slamming his clawed hands down in the snow and sending snow flying through the air.
   "Oh jeez... It's been so long since we've had to do this... What did we do last time?" Undyne mumbled, looking to Papyrus for suggestions.
   "We just kept trying to remind him of who he truly is! He'll remember eventually!" Papyrus answered with a fierce determination edging his voice.
Undyne nodded. "Sans! You have to remember! Take back control!"
The beast shook his head from side to side, his glowing red eye leaving a trail of pale red light in the dim light.
"Fight it! I know you can do it!" Papyrus shouted.
   The beast squeezed his eyes shut, dragging his claws down his face. When his eyes opened once more, his eye was a bright cyan color.
   "Wh-What happened?"
   "You were fighting your insanity. It's okay, we helped you through it," Papyrus said with a grin.
   "What's with Undyne?" Sans growled, pointing to the fish monster.
   "She snapped out of her insanity too. We all have," Papyrus replied.
   "Well, just us three. The others are still.. well, y'know..." Undyne corrected.
   "Hmph. Go figures. Oh well, at least we're all sane again, right?"
   Papyrus nodded. "You've got that right brother."
   "I love ya bro."
   Papyrus leaned into his brother's massive skull, hugging his muzzle tightly. "I love you too."

A/N: Wow, that was long! I went over 2000 words on this one! A new record! XD
Anyways, leave your requests in the comments below! :3
-Sora