This was requested by BeththeSkeleton334 . This is (sorta) a Soriel oneshot. Just to let ya guys know. :3
   Also, the drawing above is by me! You can find more of my art in my artbook! :3
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   Sans stood in the center of the snowy clearing, looking around in confusion. Which way was he supposed to go? Why was it so hard to remember these things anyway? He'd walked this path, what, countless times before?
   He shook his head, unable to make sense of his thoughts. He held a hand to his forehead, closing his eyes briefly as he took a deep breath.
   Just focus. This is nothing new. This isn't a difficult thing. Just figure out which direction leads to the door.
   He opened his eyes again, trying to refocus and find the right path. After several minutes, he figured it out, though he was unsure if it was actually the correct way or not. Oh well. If it was wrong, he could just go back to the clearing and go a different way. The clearing wasn't that hard to find anyway.
   He picked up his pace as he spotted the door off in the distance. So he was right! The skeleton smiled as he approached the door, sitting down with his back against it as soon as he got there.
   Sans exhaled slowly before knocking. "Knock knock."
   There was shuffling on the other side of the door, followed by the sound of a woman's voice. "Who's there?"
   "Ketchup."
   "Ketchup who?"
   "Nevermind. I think I'll ketchup to ya later."
   There was laughter on the other side of the door. "Hello there, Sans."
   "Hey Tori," Sans responded, his grin widening and his eyes sparkling. "How've you been?"
   "Good. And you?"
   "Eh. I've been better, I guess," Sans shrugged.
   Toriel seemed taken aback by this. "What do you mean, my dear friend?"
   Sans sighed. "I just feel a bit... off. Like, I've been forgetting things that I really shouldn't be forgetting at this point."
   "Like what?"
   "Like how to get here. I got mixed up on which direction I was supposed to go from the clearing to get here. And I never forget that."
   "Hmm. I'm... not really sure how to help... I'm sorry you've been forgetting things."
   Sans shrugged. "Eh. It doesn't matter. I'm here now, right? So let's get off this subject. I've got a skeleton of puns to tell ya."
Toriel chuckled, but it seemed half-hearted. "I've goat to be honest with you, I don't think puns are the best way of getting rid of your problems."
Sans laughed, but he fell silent as he really thought about what she'd said. "Come on Toriel. I'm trying to be funny here."
"I know. I just... I worry about you, my friend."
Sans's breath caught in his throat. "Oh who am I kidding. None of this is fine. And now I've got you worrying. I'm sorry."
"There's no need to be sorry. It's not your fault that you're not okay. Just... Don't keep trying to hide it. I feel that you tend to hide things, and then they become a real problem later on. So if you ever need to tell me anything, don't be afraid to confront me about it," Toriel insisted.
"I know.. It's just... " Sans trailed off as he felt a strange burning sensation in his chest. He gripped a hand to his sternum, looking down to see a faint blue light pulsing underneath the fabric of his shirt.
"-Sans?"
The skeleton's head snapped up at the sound of Toriel's voice. "Huh?"
"You didn't finish your sentence. Are you alright?"
"Uhh, y-yeah. I'm fine. Just lost my t-train of thought. Heh..." Sans replied, an unsure tone in his voice.
   "You do not sound okay... Perhaps you should head back home and we should talk another time-"
   "No," Sans growled, his voice echoing slightly. He took a deep breath. "I'm sorry. I just... I like being around you."
He felt his soul pound harder as he confessed. He became aware of a distinct blue color tainting the left side of his vision. He closed his left eye, though he could still see the blue light reflecting on the snow at his feet.
Sans felt the burning in his chest grow stronger. Sweat dripped from his forehead and his skull throbbed. It was getting even harder to think.
"Y-You mean so much to me. You always t-try to help me through whatever I might be d-dealing with.." Sans broke off with a shudder. "I... I love you, T-Toriel."
   For a few moments, Toriel said nothing.
"Sans... I didn't know you felt that way.."
His bones were beginning to cramp. His skull felt hot and he was becoming aware of a building sense of claustrophobia, like his body was to small to keep whatever was building up within him contained.
   "But it is quite a funny thing..."
   Sans gasped, barely managing to keep silent as pain surged through his chest.
   "Because I love you too."
   Sans's left eye snapped open as she said this. His soul was pounding so hard that he could almost hear his ribcage rattling. He opened his mouth to respond, but the only sound that came from his mouth was an agonized moan as pain tore through his chest once more.
   "Sans?! What's going on?!"
   He panted heavily, his grip tightening on the fabric of his shirt. His other hand twitched as the pain began at the tips of his fingers. He let out a grunt as the pain worsened.
   "Sans!"
   His hand twitched again, his smile faltering. The flames from his left eye began to spread further across his skull. He gritted his teeth, barely holding back another pained grunt.
"Sans, please! Answer me!"
"I-I can't..." Sans panted, his voice hoarse.
"What?? Can't what?? What's going on??" Toriel's voice was urgent.
As Sans began to respond, he broke off with a groan as the pain worked its way into his wrists. He let go of the fabric of his shirt and clutched at his hand. He felt his soul lurch as he heard the sound of bones cracking.
   His gaze snapped to his hand, his eyes widening as he realized that they were changing. The tips of his fingers had grown sharper, resembling claws of some sort.
"Wh-What the hell is happening to me?!" Sans exclaimed.
"What's wrong?? You sound like you're hurting.."
Sans cried out in agony as the pain spread further up his arms. He heard the sickening sound of bones creaking and snapping, alerting him that the transformation was still happening.
   "Oh god... It h-hurts..." Sans whimpered softly, his voice edged with a slight growl.
He screamed in pain, the growl in his voice becoming more distinct.
"You sound different... I-I'm not sure how, but something about your voice seems off..."
Sans didn't respond, taking a moment to catch his breath.
   "Sans, I'm not sure what's happening over there, but I think I'm going to come out there... You aren't really responding and it's starting to worry me.."
   Sans could faintly hear shuffling on the other side of the door, followed by the creak of it opening.
   "T-Tori, no! D-Don't come over here! I d-don't wanna hurt you!" Sans protested, surprising himself with his own words.
   She paused, debating whether she should listen to him or not. She finally made up her mind, opening the door even more, despite her better judgement, and looking around for the other monster.
   Her gaze finally rested on a small skeleton leaning heavily against the door, a pained look on his face. She raced over to him, kneeling down in front of him.
   "Sans?"
   "T-Toriel... I thought I t-told you not to come over h-here..." Sans murmured hoarsely.
"I won't leave you here to suffer alone, my dear friend," Toriel insisted, grabbing ahold of his hands with her furry white paws, her eyes narrowed with determination.
Sans cried out in agony, lurching forward into Toriel's arms. She seemed surprised, but said nothing. She could feel his fingertips digging into her paws as his grip tightened, holding back a yelp as she felt his claws poking into her flesh.
"T-Tori... Please... H-Help me..." Sans panted, his voice slightly muffled.
"It's okay.. I'm right here. I won't leave you," Toriel murmured.
Sans groaned in agony as the pain spread to his spinal area. He arched his back, leaning heavily on Toriel as large spikes slowly began to push up through his vertebrae. The agony was unbearable.
The spikes started to press against the fabric of his jacket, forming a ridge that ran down the length of his spine. The fabric continued to stretch before finally tearing, the sound of bones cracking becoming more apparent now that there wasn't much to muffle the sound.
Sans felt nauseous at not only the sound of his bones snapping, but just the mere thought of what was happening. Everything about this was just so... sickening. It wasn't right.
It felt like something was forcing him to do this against his will, but it wasn't something external that was forcing it. No. It was something far more internal that was causing this.
He relaxed for a second as the pain somewhat subsided. He pulled away from Toriel slightly, just enough to look into her eyes. Though there was a fearful gleam to them, they were flooded with perseverance. No matter what, she was going to stay by his side.
Sans slowly became aware of a strange tingling sensation at the base of his spine. He tried to ignore it, trying to focus on Toriel instead of what he was feeling.
"T-Tori... Y-Ya gotta p-promise me something, alright?" Sans murmured, his voice rough and almost double-toned.
"Yes?"
"If s-something goes w-wrong, and I lose myself, p-promise me you'll get o-outta here..." Sans continued, his soul lurching as he realized that his voice sounded slightly deeper than normal.
Toriel didn't respond. Sans could feel that weird sensation from before growing stronger, and it took everything in him not to look behind him. He just needed to focus on Toriel.
"T-Tori, please..." Sans pleaded, the tone in his voice growing more desperate.
"Sans.. Have you always had a tail??" Toriel asked.
"Wh-Wha?" Sans finally turned around, gasping as he realized that a tail had begun to grow from the base of his spine.
"What's happening to you??"
"I don't- look. i-it doesn't matter! I need t-to know that you'll leave i-if things get b-bad. Please!" Sans begged.
Toriel hesitated. "Alright. But what if-"
Sans cut her off with a cry of anguish as the searing pain suddenly flared up within him once more. He fell back into Toriel's arms, his breaths coming in short, ragged gasps as heatless blue flame poured from his left eye and rapidly spread across his entire body.
He held onto the goat monster's arms for support, digging his clawed fingertips into her sleeves, completely unaware of the pain he was causing her. He felt like his spine was slowly being stretched out, adding to the pure agony he felt.
A strained moan forced its way out from between Sans's sharpened teeth as pain shot through his legs. He heard the nauseating sound of bones cracking and shifting once more as his feet suddenly began to elongate, the tips of his toes growing sharper and ripping through his worn-out slippers.
Sans let out another gut-wrenching moan as his joints slowly cracked back on themselves, the structure of his legs shifting from plantigrade to digitigrade. His tail twitched under the strain of his transformation as it continued to grow longer.
Sans began to get a weird sense that if he continued to lean on Toriel, he'd crush her, so he reluctantly pulled away from her, choosing instead to use the door behind him for support.
"Sans?!" Toriel exclaimed, reaching out to the skeleton.
Sans cried out in agony as everything started rapidly changing all at once, though the voice that tore from his throat was definitely not recognizable as his own.
He tried to stand, his legs shaking as he dug his claws into the door, dragging them down and leaving huge clawmarks on the wood. He shook his head violently from side to side, clutching a hand to his skull while snarling ferally.
   His face began to stretch out into a muzzle as horns emerged along the back of his skull. He screamed as his face split from his nose socket to the center of his forehead. Toriel flinched at the sound, unsure of how to help her friend.
   Sans turned his head to look back at the goat monster, an almost tired look in his eyes. Suddenly, his pupils became slits as pain tore through him once again, a howl of agony rumbling from deep within his throat.
   Twisted bone shards shot up through Sans's shoulder blades, tearing through what was left of his jacket as his ribcage expanded. He dropped down onto all fours as the last of the changes took place.
   For a moment, Sans just stood there, clawing at his skull and growling softly. Toriel was completely still, unsure of how he'd react to her trying to do anything.
   "...Sans?"
   The beast's head snapped up, his cat-like eyes fixated on the goat monster. A look of fear crossed her face as the skeletal creature stalked towards her.
   "Sans, it's me, T-Toriel..."
   Sans snorted and tossed his head, his tail flicking. Wisps of blue smoke coiled out from between his long, curved fangs as he let out a low snarl.
"Please... Don't do this..." Toriel murmured, backing away from the beast slowly.
The beast continued to advance towards the goat monster, no sign of recognition in his eyes. It was almost as if he were being controlled by something else... something unseen...
   Suddenly, he lashed out, giving Toriel barely enough time to dodge. She instinctively darted to the side, a ball of fire magic forming in her paw. Sans skidded to a halt, sending snow flying in the direction he had just been going.
   He stared at the flames that Toriel held in her paws, a curious, yet suspicious look on his face.
   "I don't want to fight you. But if I must, then I will," Toriel stated firmly, though her voice shook with fear.
Sans rumbled in amusement, tilting his head to the side. Toriel's gaze hardened at the beast's response. "What's so funny?"
A grin spread on Sans's face, though it was not a friendly one. In fact, it was quite far from it. It was more crazed than anything.
Stop! This is not me! I would never attack Toriel! Or anyone, for that matter!
   The beast shook his head as his thoughts screamed at him to stop. He felt a horrible burning sensation gathering in his throat as magic built up, forming a ball of pure energy in his maw. A look of terror flashed across Toriel's face.
   Suddenly, against his own will, a beam of pure magic burst from Sans's mouth, barely missing Toriel. As a last resort, the goat monster hurled a fireball at the beast's face. It hit him directly between his eyes.
Sans staggered backwards, roaring in a mixture of pain and fury while clawing at his face.
"Listen! I don't know what's gotten into you, but you have to stop! This isn't the real you! The real Sans is a pun-loving, laid-back skeleton, not this crazed beast!" Toriel shouted over Sans's roaring.
   Sans's gaze snapped back to the goat monster, his eyes gleaming with pure rage. A growl rose up in Sans's throat as another beam of energy built up in his maw.
   Toriel darted swiftly between Sans's legs, startling the beast and causing him to lose his aim, which ended in a gaping hole being blown into the door.
   Sans roared, rearing up on his hind legs before slamming his clawed hands back down onto the snowy ground below. He whipped around, his gaze fixated on Toriel once more.
   "Attttaaackkkk......" he rumbled, his voice so much deeper than normal that it didn't sound like himself at all.
   "No. No more attacking. You need to calm down. Remember who you are," Toriel insisted, a determined tone in her voice.
   Sans lowered his head, his piercing blue gaze locked onto the other monster. She flinched, but said nothing. He moved his head closer, his muzzle nearly touching Toriel.
   Hesitantly, Toriel reached out and put a paw on his forehead. Sans twitched, surprise sparking in his eyes. He rumbled quietly, though this sound was more like a purr than an aggressive growl.
    The beast closed his eyes, his breathing steadying. When he reopened his eyes, his pupils had returned to normal and his eye color had gone back to its original white color.
   "T-Toorrrrrrielllll?" Sans stammered, as if trying to talk for the first time.
   Toriel chuckled, brushing a paw down the crack that split the skeleton's massive skull. "What have you gotten yourself into, my friend?"

A/N: Sorry this took awhile longer than I thought! I wrote a lot of this in one sitting, and then I got temporary writer's block! -_-
But it's finally done! And I had fun doing it! :3
I will be halting requests temporarily, just so that I can get caught up on the ones I already have. :P
For now, stay determined!
-Sora