Not a request. As I mentioned in the previous part, I've been wanting to do a oneshot with lust sans. welp, enjoy the suffering of the slutty sans. XD
   Also, Lust Sora will be making her first appearance ever in this one. I couldn't think of another character to include so... yeah... don't judge.
WARNING! DO NOT READ IF YOU ARE UNCOMFORTABLE WITH SEXUAL REFERENCES! THERE WILL BE IMPLIED SMUT AND SOME MATURE LANGUAGE! YOU'VE BEEN WARNED!
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   Sans chuckled as he flopped over next to Sora on the bed. Sora gazed back at him, a grin on her face.
   "That was fun," Sora purred.
   "We should do it again sometime," Sans panted.
   Sans sighed as he stared up at the ceiling. After several minutes of just sitting there, Sans started to fall asleep. Sora did the same.
   Sans had barely been asleep for a few minutes before he was jolted awake by a sharp pain in his chest. He shot upright on the bed, clutching a hand to his sternum in response to the sudden pain.
   Sora shifted slightly on the bed, but didn't wake up. Sans looked over at the sleeping angel dragon, making sure she was truly asleep before he got up. He didn't want her to worry about him.
   He stumbled to his feet and quietly exited the room. He made his way to the bathroom, which was luckily just down the hallway. Once he got there, he turned on the sink and splashed water on his face, trying to bring himself back to his senses.
   He stared at his reflection in the mirror, noticing for the first time that there was some sort of purple liquid trickling from his eye sockets, almost like he was crying.
   He wiped his left eye with his hand and stared at the strange purple substance on his hand.
   "What is this? Why is it leaking out of my eyes?" Sans mumbled.
   His gaze trailed down to his chest, which for some reason had a faint purple light pulsing underneath the fabric of his shirt. Puzzled, Sans lifted up his shirt, exposing his ribcage.
   He realized that the light was coming from his soul. But why??
   He stumbled back towards his room, his mind reeling from what he'd seen. His footsteps were loud, waking up Sora once Sans had gotten back to the room.
   "Sans? What are you doing? Is something wrong?" Sora asked.
   Sans jumped at Sora's voice, not expecting the angel dragon to actually be awake. "I'm fine! N-Nothing's wrong!"
   Sora gave him an unconvinced look. "You don't look okay. Tell me what's wrong."
   Sora got up and walked over to him, reaching out to grab his hand. Sans smacked her paw away and backed up against the door. Sora looked taken aback.
   "I told you, I'm fine! S-Stop worrying so damn much!" Sans snapped.
  Magic started to leak from his eyes once more, the strange purple liquid dripping onto the floor and pooling at his feet.
   "What is that?"
   Sans pulled his hood up over his face, the cyan-colored fur lining his hood covering his eyes. Sora narrowed her eyes as she attempted to yank his hood back down, but Sans held it firmly in place.
   "Sora, stop," Sans growled, his left eye flaring briefly with a bright purple light.
   Sora flinched and let go of Sans's hood. The two monsters averted eyes for a few moments before Sans suddenly doubled over with a grunt as pain surged through his chest once more.
   Sora's head snapped up in response to the sound. "What's wrong?"
   "Nothing! I've said it over and over! I'm FINE," Sans insisted, his grin twisting into a snarl.
   Sora snorted in disbelief. "And I keep telling you that I know you're not fine! So stop being so childish about this! If you're hurt, then I can help you-"
   "Stop! I don't wanna talk about it!"
   Sora opened her mouth to respond, but was cut off as Sans let out another grunt, this one louder than the last.
   "Look, let's just forget about this. I'm fine, end of story," Sans mumbled, his voice somewhat strained.
   The skeleton stumbled over to the bed, trying to get a flirty grin to stay on his face as he laid down on the bed in a seductive pose. But, even though he tried to hide it, the forced smirk on his face didn't do much to hide the pain he felt.
   He stuck out his tongue seductively, trying to convince Sora that nothing was wrong with him. His expression nearly broke as he felt another jolt of pain in his ribs.
   "No. We're not gonna play this 'sexy game' right now. And even if you are, then I'm not. You are in obvious pain, and no matter how much you tell yourself that you're fine, I can tell that you're not. Stop lying to yourself," Sora persisted.
   Sans's grin faltered for a split second as he tried not to let his pain show. "I'm not lying to myself. I really am fine."
   He gritted his teeth, grasping his ribs tightly with one hand as pain surged through him once more. Sweat trickled down his skull as he strained to keep himself from crying out in agony.
   "Ok, maybe instead of trying to seduce me right now, you should go outside for a few minutes. Try to get your head straight, m'kay?" Sora suggested.
   Sans inhaled sharply, his grin briefly becoming a grimace. "Okay... Whatever ya say, mommy~"
   Sans slowly got up from his seductive pose, clumsily slipping on his boots before stumbling over to the door. He struggled to turn the doorknob, but eventually managed to open the door.
   "I won't be long," Sans promised before closing the door.
   He huffed slightly as he tried to get to the stairs. He had barely made it three steps before a wave of pain overtook him, causing him to collapse to the ground with a loud thud.
   The sound got Sora's attention. The snow-white angel dragon threw open the door and rushed to the skeleton's side.
   "What happened?" Sora asked, concern sparking in her violet-colored eyes.
   Sans panted heavily as he tried to get back up. He felt a paw on his shoulder as Sora tried to comfort him.
   "You're really shaky... Do you need help getting down the stairs?" Sora offered.
   "N-No... I can manage..." Sans insisted, his voice weak.
   "Too bad. I'm helping you, no matter what you say," Sora growled.
   Sans let out a cry of agony as another wave of pain wracked his body, causing him to kick out his legs and knock Sora back with a solid blow to her gut.
   Sora recoiled back, holding a paw to her midsection in the place that Sans had kicked her. Sans tried once more to get back on his feet, but he fell back down as pain shot through him once more.
   He rolled over onto his side, moaning in agony as the pain in his bones grew worse. He curled up into a ball, one hand keeping his hood pulled down and the other firmly clutching his chest.
"Ok, I'm not fucking around anymore. I'm taking you outside, whether you like it or not," Sora grumbled, picking Sans up bridal-style and racing down the stairs.
Once she had made it to the bottom, Sans's body was wracked with another wave of pain, causing him to whimper quietly. Sora rushed outside as fast as she could, which was considerably slower than usual due to Sans's added weight.
"Hmmm... Where should I go now?" Sora mumbled, trying her best to tone out another moan of pain from Sans.
"The woods maybe? No one would find us there... Okay, then that's where I'll take you," Sora muttered under her breath.
Without wasting another second, Sora took off in the direction of the forest, her pace slowed by the weight of having to carry Sans.
Sora suddenly let out a yelp as she felt something sharp dig into her arm. She looked down to see that Sans was tightly gripping her arm. She stopped and took a second to look at his hand.
Her soul skipped a beat as she noticed that the tips of Sans's fingers were somehow growing sharper.
"Oh god... This is worse than I thought... Just hang in there Sans... I'll figure something out..." Sora murmured as she continued onwards into the forest.
Sans's only response was a strained moan as pain tore through him once more. Sora picked up her pace as she heard the muffled sound of bones cracking.
As they got further into the woods, the pain seemed to keep getting worse for Sans, his moans of agony seemingly getting louder and louder.
Eventually, they made it to a clearing deep within the forest. Sora carefully put Sans down in the middle of the clearing and sat down next to him, gently stroking his skull in an attempt to comfort him.
"Oh god... Sora, it hurts... It hurts so much..." Sans whimpered, his voice hoarse.
Sora shuddered as she heard the sound of bones cracking once more, instinctively shutting her eyes so that she couldn't see the pure agony that Sans was going through. It was no use though. She could still hear the skeleton moaning in pain despite her attempts to shut it out.
Sora was snapped out of her thoughts as Sans grabbed onto the fabric of her sweater, his sharpened fingertips slowly tearing the fabric.
"S-Sora, please... Help-" Sans cut off with a wail as pain surged through him, causing him to lurch forward.
He barely managed to catch himself before his skull hit the ground. He began to shudder violently as he slowly lost control of his own mind. He couldn't think. Even if he wanted to, the intense pain in his body prevented him from doing so.
Sans suddenly let out a scream of agony as pain shot down his spine. He arched his spine as he felt his spine begin to re-set itself with a series of dry snaps. The pain was unbearable.
Sora's attention snapped to the skeleton, her eyes widening in shock as she noticed the back of Sans's jacket beginning to bulge. She flinched as the purple fabric eventually tore, revealing the massive spikes that had formed along Sans's vertebrae.
With what little strength he had left, Sans managed to look up at Sora. The gleam in his eyes showed a mixture of pain and fear as his gaze locked onto Sora's.
"Sora... You need to leave... N O W ," Sans panted, his voice strained.
"I'm not leaving you!" Sora insisted.
Sans grunted in pain, tightly grasping his ribs with one arm. "G O N O W !"
Sora flinched at the slightly animalistic tone that now edged Sans's voice. Reluctantly, Sora ran off into the forest, back in the direction of Snowdin. But, she darted behind a tree on the outer rim of the clearing, determined to make sure that Sans was going to be okay.
Much to her surprise, Sans had managed to stand up. He stumbled to the other end of the clearing, barely making it to the other side before another wave of pain wracked his body, causing him to double over with a groan of agony.
He dug his sharpened fingertips into the bark of the tree, desperate to stay on his feet. Despite his attempts, Sans fell down on his hands and knees as the pain worked its way into his legs.
He could feel his feet begin to change too, the joints cracking back on themselves as his feet elongated and the tips of his toes grew sharper. His boots were completely destroyed by the time his feet had stopped changing.
His jacket and pants were in tatters, much like his boots were. Sans knew it was still going to get worse, since he had a strange feeling that the transformation was only just beginning.
He gritted his teeth as another wave of pain shot through him. He could feel a strange tingling sensation at the base of his spine as he somehow began to grow a tail.
Sans threw his head back as he wailed in agony. He could feel the transformation beginning to affect the shape of his skull, his face stretching out into a long muzzle and his teeth growing sharper.
   Small bumps were beginning to emerge along the back of his head, rapidly erupting into curved horns shortly after they had appeared. Sans screamed in agony as his skull suddenly split from his nasal cavity to his browline.
   He was taken aback by the sound of his voice, which sounded more like an animalistic howl rather than his normal voice. It was almost completely unrecognizable.
   After what seemed like forever, the transformation stopped. Sans simply laid there in the snow, afraid that any movement would bring the pain back. His breath rattled strangely in his expanded ribcage as he tried to calm down.
   Sora peeked out from behind the tree, her violet-colored eyes sparkling with concern. She was afraid to call out Sans's name, out of fear that he wouldn't be in control of his mind.
   Finally, she snapped out of her timidity.
   "Sans?"
   Sans's head snapped up at the sound of Sora's voice. His gaze pierced into the angel dragon's soul, his glowing purple eyes fixated on her. Recognition flashed in his eyes before suddenly, his usually heart-shaped pupils became slits, giving his eyes an almost cat-like appearance.
   His friendly grin twisted into a vicious snarl as he slowly rose to his feet and began to stalk towards Sora. Sora whimpered as her ears flattened against her head. Without thinking, she spread her wings and fluttered up the tree.
Sans roared as he charged at the tree, the heavy impact of his skull ramming the trunk causing the tree to fall down. Sora screeched as she fluttered to a different tree, her eyes wide with terror.
Sans stepped on the fallen tree, the trunk easily snapping as though it were a small twig. Sora flinched, knowing what this beast that Sans had become could do to her if she was caught.
Sans looked up at the branch that Sora was perched on, a primal gleam in his glowing purple eyes. Sora held her breath as her gaze locked with his for a heart-stopping moment.
A low growl rumbled from Sans's throat, the sound quickly escalating to a loud snarl as he suddenly lunged forward and clamped his powerful jaws down on the trunk of the tree.
Sora gasped as she felt Sans begin to yank the tree out of the ground, the terrified angel dragon clinging onto the branch with all the strength she could muster.
"Sans! Stop!" she screamed.
Sans tore the tree out of the ground, causing Sora to scream in pure terror. He dropped the tree and crept over to where Sora was still desperately clinging to the branch.
She stared up at the huge beast, fear sparking in her shimmering purple eyes. Somehow, the fear in her eyes reminded him of who he really was.
He backed away, clutching a clawed hand to his skull as he growled softly, shaking his head side-to-side as he fought for control of his mind. After a few moments, Sans reopened his eyes, the feral gleam gone and his pupils reverted back to their usual heart shape.
   "Sora? Wh-What happened?"
   "You went crazy and tried to attack me," Sora responded.
   "Huh? Why-" Sans stood up awkwardly on his back legs and looked down at himself. "Where are my clothes?! I thought I put them back on before I went to sleep last night!"
   "You did... But you turned into, well, that," Sora explained, gesturing to Sans. "and yeah... Your clothes got completely destroyed."
   Sans looked confused, but didn't question the snow-white angel dragon. "Whatever ya say, vertebae~"
   Sora blushed at the nickname. At least Sans could still flirt when he was like this.

A/N: Gosh! Sorry this took so long! I've been kinda busy the past few days, but I'll try to update again soon! I already have 2 or 3 other requests that I can work on. :3
Oh jeez... Almost 3000 words?! New record! XD
Anyways, feel free to leave other requests in the comments below!
-Sora