Requested by WOmagicHO
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   Sans stared drearily at the monsters around him. He was slumped on the couch in the living room of his and Papyrus's new house on the surface. He held a video game controller in his hands, but he wasn't even paying attention to the game.
   Slowly, the voices of his friends around him faded away as he drifted into sleep, the controller slipping from his grasp and falling to the ground.
   For a few moments, all Sans could see in his mind was a pitch-black abyss. Then, the darkness faded away, leaving him standing in the Judgement Hall. His soul pounded in his ribcage as he looked around.
   That was when he heard it. Voices. No, that wasn't right... it was just a single voice, calling to him from the darkness beyond his own vision.
   "Hey Sans~"
   Sans flinched as he recognized the voice. His hand came to his neck and he jumped as he felt the dust-coated fabric of his brother's scarf around his neck. He looked down the hallway, his gaze locking into the red glare of none-other-than Chara.
   "Funny, isn't it? You're always too late to save the ones you love. Even when you do succeed, it all gets snatched from you as the world resets. It's like you're never meant to have a happy ending in the first place," Chara sneered, a cruel grin spreading on her face.
   "Please... Stop..." Sans murmured, his hands clutching at his skull.
   "What? Stop what? Simply stating the truth?" Chara continued.
   "Nrgh... Pleassse..." Sans growled, falling to his knees as he pressed the palms of his skeletal hands to the sides of his head.
   "Does it bother you? To know the truth?"
   Sans had stopped responding at this point. Chara giggled as she stepped closer to the skeleton.
   "You can't escape fate. It always finds you~" Chara snickered, raising her knife to strike.
   Just as the knife came across Sans's ribs, he was jolted back into reality. He awoke with a scream, his eye sockets flying open.
   "Sans?"
   Papyrus had noticed him.
   "What's wrong, brother?"
   Sans was still slumped over on the couch, but his breathing was escalating rapidly. He clutched a hand to his chest. A burning pain was beginning to build up inside of him. Sweat was beginning to trickle down his skull. His left eye was blazing with a bright icy-blue flame.
   His soul pounded and tore at him from the inside. It was becoming unbearable. His entire body felt like it was on fire.
   Suddenly, he lurched forward with a grunt, falling off the couch in the process. The telltale sound of his bones beginning to crack was the only warning he needed.
   Sans tried to stand up, but immediately fell back down as his spine began to snap and twist while massive spikes began to form along his vertebrae. He arched his spine with a series of dry snaps as a jolt of pain was sent through his spine. A pained groan forced itself out from between his clenched teeth, which were rapidly growing sharper.
"Oh my... Sans?" Toriel murmured as she rushed to the skeleton's side.
Frisk followed Toriel, a concerned expression on their face. Alphys hurried over as well, leaving Undyne mid-sentence.
"What the hell, Alph?!" Undyne shouted, trailing off as she saw Sans.
A few feet away, Asgore was staring at Sans with a look of both confusion and fear in his eyes. He slowly began to back away, carefully watching to make sure he didn't trip on his cloak.
Sans let out another groan as his feet began to splinter and shift. The tips of his toes were growing sharper and were beginning to pierce through his slippers.
Papyrus rubbed his brother's skull gently, wincing every time he heard a bone snap.
"It's okay brother... You got this..." Papyrus murmured.
Sans dug his fingertips into the ground and dragged his hands towards his body, leaving heavy clawmarks on the floor with his newly-formed claws. Sans moaned in agony as his bones snapped and reformed, each individual fracture sending a wave of unbearable pain through Sans's entire body.
His legs were beginning to extend into digitigrade, making the once-small skeleton appear to be much taller than he usually was. Sans felt a strange tingling sensation in the lower portion of his spine as he began to grow a tail.
Frisk visibly winced as another gut-wrenching moan escaped from between Sans's clenched teeth. The skeleton's voice was beginning to change; it was starting to double-tone and turn into growls as well as horrific screams.
Before long, the transformation began to die down. After several agonizingly long minutes, a massive skeletal beast loomed over the three monsters and the human child that had been at his side only moments before.
Undyne summoned a glowing spear in her hand as she stared at the huge creature that Sans had become. Although she tried not to show it, deep down, she was outright terrified. Her single yellow eye narrowed in determination. Her mouth twisted into a fierce snarl, exposing her slightly yellowed fangs.
"I'm not afraid of you, Sans," Undyne hissed.
"Undyne, no! It's okay! He's not going to hurt anyone!" Papyrus shouted, his dark eye sockets flooding with worry.
"How can we be sure?" Undyne growled.
"Papyrussss..." Sans rumbled, his blazing left eye showing his confusion.
"See? Told you! He's completely safe to be around!" Papyrus insisted.
After a few moments, Undyne let her spear dissipate. There was no point arguing with Papyrus.
He was Sans's brother, after all.

A/N: Wow! I actually wrote a long chapter for once! (well... longer, I guess... the chapters for this book are a bit longer anyways. :/)
Anyway, make sure to leave your requests in the comments below! As I've said before, I will do pretty much any au and OCs are welcome!
Also, it's preferred if you would come up with a scenario for requests. All the requests I've gotten have given me scenarios, but for future requests, it is a hell of a lot easier to write when I have a scenario. ;3
Thanks!
-Sora