This was requested by izz2772 .
   This one isn't as focused on the transformation aspect of the au, but rather the backstory behind it. This is my take on that backstory.
   It is heavily based off of the backstory of "Best of Times, Worst of Times", which is an AMAZING gaster blaster au fanfic written by abadtime on tumblr. I highly suggest that you guys check that out. It's actually one of my favorite undertale fanfics of all time! (it's also on AO3, and is much easier to read that way. Tumblr fucked up her entire blog and now it's hard af to read the damn fanfic. :/)
   Anyways, enough of my rambling. On with the oneshot! >:3
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   Papyrus jolted awake as he heard a muffled scream from Sans's room. He rubbed the sleep from his eye sockets before quietly getting out of bed, carefully making sure that he didn't wake up Frisk, who had spent the night and was sleeping in his bed.
   Luckily, the child didn't even flinch as Papyrus got out of the bed. The taller skeleton moved silently towards his door and opened it as silently as he could. He slipped out into the hallway stealthily, successfully making his way out of the room without waking up Frisk.
   For a moment, Papyrus stood outside of his brother's room, debating whether or not it was smart to go in. Chances were, it was just a night terror. But what if it was something much worse?
   Another scream on the other side of the door made Papyrus's decision for him. He threw open the door, his gaze resting on the shuddering form of his brother curled up on the floor.
"Sans? What's wrong? Is it the night terrors again?" Papyrus asked, his gaze flooding with concern.
Sans grunted in pain before he could even say anything. "Pap, you need to leave..."
"No. Something is wrong with you. I will not leave when my brother is in obvious pain," Papyrus said firmly, reaching out and resting a hand on Sans's shoulder.
Sans instantly slapped his brother's hand away, giving Papyrus a chance to get a glimpse at the smaller skeleton's hand.
Papyrus gasped as he noticed that the tips of Sans's fingers had grown sharper. "Sans?! You're turning?! Right now?!"
Sans let out another grunt, this one louder than the last. "Yes! What does it look like?!"
Papyrus flinched at the surprisingly fierce tone in Sans's voice. "Okay... What can I do to help you?"
   Sans choked down another grunt, clenching his teeth as pain tore through him. "Nothing! Y-You can't stop it once it starts!"
The two skeletons were completely unaware of the snow-white angel dragon that was standing in the doorway, her blue-green eyes sparking with concern at the sight of Sans in pain.
Eventually, Sans noticed the worried angel dragon in the doorway. "S-Sora... Please... You need to get outta here..."
Papyrus turned to see Sora in the doorway, a look of surprise on his face. Then he remembered that she had spent the night as well, sleeping on the couch as opposed to sleeping with either of the skelebros. She personally didn't like sleeping with others in the same bed.
"No! Sans is hurting! Sora not leave Sans to suffer alone!" Sora insisted, a determined tone entering her voice.
   Sans let out another scream as pain shot down his spine. The sounds of fabric tearing and bones cracking tore through the silence of the room as massive spikes began to push through the fabric of Sans's jacket.
"Guys, seriously. I can handle this-" Sans cut off with a wail of agony as pain shot through his legs.
The sound of bones snapping filled the air once more as his feet began to change, the joints cracking back on themselves as his feet elongated and the tips of his toes grew sharper.
A tail began to grow from the base of Sans's spine as his legs began to grow longer. His skull was beginning to change as well, his face elongating into a muzzle as his teeth grew sharper and horns erupted from the back of his skull.
When all was said and done, Sora and Papyrus were left staring up at the beast that Sans had become. The sounds of the transformation must have woken Frisk up, because the human child was standing in the doorway with a terrified look on her face.
Sans noticed her and tried to make himself less intimidating. "It's okay kiddo. I won't hurt you. It's safe to come in."
Frisk hesitantly came into the room, a nervous expression on her face.
"Sora always wonder how Sans become like this," Sora murmured.
"You really don't wanna know. It's... not exactly a pleasant story..."
"Well then getting it off your chest might make you feel better," Papyrus responded.
"I'm kinda curious too..." Frisk added quietly.
Sans sighed. "It's kind of a long story... Ya still wanna hear it?"
All three of them nodded.
"Okay... Here goes... You all know how me and Paps were created in a lab?"
Sora and Frisk nodded. Papyrus didn't really need to give his response, being that he was one of the mentioned subjects.
"Well, the scientist that created us wanted to create a weapon. One that could break the barrier and scare the shit outta the humans. I was destined to be that weapon of his."
"Did Sans know about scientist's plans?" Sora asked.
"Well, yeah. He'd told me his plan and I was completely on board with that. Being a living weapon sounded cool. I was a kid at the time, anything like that sounds cool."
"Soo you wanted to be like this?" Frisk pressed.
"At the time, I was completely unaware of the pain I would have to go through for this to work. Gaster had never told me about how much it would hurt to transform. But what the hell did I expect?! My bones are literally snapping and reforming! It hurts like hell!"
Everyone fell silent as Sans ranted.
"Anyways, to get to the point where I could become this 'weapon' of his at will, I had to go through constant testing. About every few weeks, I'd have to get several injections of determination. And let me tell ya, that shit burns. It feels like your entire body is on fire and you can't do anything to put it out."
Sans sighed as he looked away. "It's tolerable for a few seconds... Until it reaches your soul. I can't even describe how agonizing it is when it reaches your soul."
Sora chirped sympathetically for her friend. Papyrus and Frisk stayed completely silent.
"Then, one day, we finally completed the project. It was shortly after the final injection that I began to notice that something was... off..."
Sora perked up curiously.
"It burned like usual, but as soon as it reached my soul, something just didn't feel right. The burning was gradually becoming an actual pain... Like someone was trying to crush every single bone in my body all at once."
Papyrus visibly winced at the thought of that kind of pain.
"Then... The transformation came out of nowhere. I didn't expect it at all. Gaster knew, but he hadn't told me that it would be this round of injections that would cause me to transform. He had only told me that it was going to be soon."
Frisk frowned as she listened quietly.
"Worst part about the first transformation was that it was so agonizingly slow. And it hurt so much more back then than when it happens now. It still hurts like hell, but the first one makes later ones seem like nothing at all."
Sans's audience looked upset for him.
"Just to give you an idea of how horrible it was, just imagine someone twisting your arm at such an agonizingly slow rate that pain just keeps jolting through your arm every second. And it's not just a slight pain, I mean like you broke a bone type of pain."
All three of them cringed at the vivid description that Sans gave. He chuckled in response, his eye sockets lighting up with amusement.
   "Geez. And I thought I was the one who didn't like hearing those kinds of descriptions. Guess I found something that I can tolerate more than you can," Sans rumbled.
   "Don't rub it in our faces!" Papyrus scolded, his face turning bright orange with embarrassment.
   "Heh heh. Y'know I wouldn't do that to ya bro. So anyways, that's pretty much how I ended up like this. I won't go in detail with the transformation part of it. Now ya know. Are you guys happy now?"
   The three of them all nodded in response.
   "Good. Now, I'm gonna get some sleep. Transforming takes a lot of energy, ya know?"
   "I understand. We'll leave you alone for awhile then. Rest well, brother," Papyrus replied, turning around and leaving the room, taking Frisk with him.
   Sora threw her arms around Sans's leg and gave him a hug before she followed Papyrus out the door. Just as she reached the doorway, she turned around and smiled at Sans.
   "Goodnight Sans."
   Sans smiled back, somehow managing to look non-threatening as his long, sharp fangs were exposed as he grinned.
   "Goodnight, my little angel~"

A/N: There ya guys go! That's my take on how Gaster Blaster Sans became the way he is. Hope you guys enjoyed it!
Make sure to leave requests in the comments below! I already have a few other requests that I'll be working on.
-Sora