Ok, I know there's been a lot of baby sans and dadster on this thing, but this time, it's gaster transforming, not sans. >:3
And no, I didn't draw this. I wish I had lol.
This was requested by izz2772
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   Gaster stared down at his lab notes, an irritated snarl on his face. He clutched a hand to his skull, trying to make sense of what he had written down.
   Something about what was written on the paper seemed... incorrect. He knew it wasn't right, but he couldn't figure out why it was wrong.
   "You okay Dad?" Sans asked, walking over to his father with a concerned look on his face.
   Gaster sighed. "I'm fine. I just can't figure out where I screwed up on these notes..."
   "Huh. What're they about?" Sans said curiously, trying to peek over the scientist's shoulder.
   "Human souls. I'm trying to figure out how to break the barrier without the use of seven human souls," Gaster responded.
   "How would you do that?"
   "Well, you see, if I can figure out how to create an artificial soul that could pass as human, then we may not need to wait for seven separate humans to fall. We could just create the souls ourselves instead of waiting for them to come to us."
   Sans nodded. "Makes sense."
   Silence fell over the two skeletons before Sans finally spoke. "Umm, Dad? You're sweating a lot..."
   Gaster flinched, taking a moment to look away from his notes and wipe his forehead with his hand. Sans was half-right; something was trickling down his skull, but it wasn't sweat.
   Gaster's non-existent stomach lurched as he realized what was going on. The reason that he couldn't focus.
   How had he forgotten?! He had a very strict schedule for this type of thing, and somehow, he had totally forgotten.
   "Uhh... Sans, stay here. I've gotta go do something..." Gaster huffed as he got up from his chair, his hip slamming into the edge of the desk as he rushed to get out of the room.
   "What are you doing?" Sans asked, his head tilting to the side like he was a confused puppy.
   "Doesn't matter. Just don't follow me," Gaster panted, struggling to open the door.
   "But-"
   "I mean it! Stay here!" Gaster snapped, his eye flaring purple for a split second.
   Sans flinched. "Okay... Just don't get hurt..."
   Gaster turned away. "You don't know the half of it..." he mumbled quietly.
   Just as he tried to open the door once more, Gaster felt a sharp pain in his ribcage. He clutched a hand to his chest, grunting as the pain seared through him. It was beginning. He didn't have much time.
   "Dad?!" Sans exclaimed as he rushed over to Gaster's side.
   Gaster tried to gently push the smaller skeleton away. "No Sans! Stay away!"
   Sans looked taken aback. Before he could say anything, Gaster let out another grunt, collapsing to the ground as another wave of pain surged through him.
   Sans was about to grab his father's hand to help him up, but he stopped as he noticed that Gaster's body almost seemed to be... melting?
   Sans felt his soul pound harder as he heard the sound of bones snapping. He could see Gaster's lab coat beginning to melt away at the seams, causing a strange white liquid to drip onto the ground below.
   Gaster let out another grunt of pain, gritting his teeth together as he tried to fight the transformation he knew was inevitable. He felt his bones begin to snap and reform, each fracture sending a wave of horrible agony through his body.
   He could feel his teeth growing sharper as his face began to elongate. Several bumps emerged along the back of his skull, rapidly erupting into massive horns as another scream of pain escaped from between Gaster's clenched teeth.
   His eye was blazing with a bright purple flame as he stared at Sans. The smaller skeleton was frozen with shock, his eye sockets wide as he watched the transformation in silent horror.
   Gaster panted heavily as the pain grew duller for a few seconds. It wasn't long before pain tore through his body once more, causing him to cry out in agony.
   There was a surge of pain through Gaster's spine as large spikes began to push through the partly-melted fabric of Gaster's lab coat. He could feel his spine snapping, twisting, and distending all at once. It was pure agony, to say the least.
After several horribly painful minutes, the transformation ended. Sans stared up at his father, his eye sockets wide and completely devoid of light. He was outright terrified of the creature that loomed over him.
   "D-Dad?" Sans stammered, his voice shaking uncontrollably.
   Gaster's purple gaze snapped to Sans, the smaller skeleton flinching at the sudden movement.
   "I though I told you to stay away," Gaster rumbled as he stared down at his son, a dull gleam in his blazing purple eyes.
   "I couldn't stay away when you were in such obvious pain. As much as I hated ignoring your commands like that, I had to make sure you were gonna be okay," Sans responded, his gaze fixated on the toes of his sneakers.
   "Hey. Look at me," Gaster said, reaching towards Sans and gently tilting his head upwards. "I'm fine. I am completely used to this. This has been happening for as long as I can remember. So don't worry about me, okay?"
   Sans nodded slowly. "M'kay. It's just... It sounded like it really hurt you... I just wanted to make sure it wasn't too much for you to handle by yourself."
   Gaster tilted his head to the side curiously. "What do you mean by that?"
   "Look, if you ever want me to... y'know, stay with you if... well, I guess it'd technically be when this happens again..." Sans trailed off mid-sentence.
   Gaster chuckled. "It's okay. I understand what you were trying to say. I might take you up in that offer someday."
   Sans smiled as he walked forward and hugged Gaster's leg. "I'll be here anytime you need me, okay?"
   Gaster smiled down warmly at his son. "Okay, my little baby bones."

A/N: Awww! Another cute one!
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-Sora