An Oath of Mercy || UnderFell Sans x Reader

thirteen

Warning: the climax gets a little graphic (nothing insane). If you've read this far with no problems, you'll be fine :D 

Sans wasn't sure how long he spent exhausting his anger, but by the time he felt mildly better, he wandered back into Snowdin town. What he didn't expect was for Dogaressa to run up to him, yipping about Papyrus. Before he could get in a solid question, she pulled him over to where the other townsfolk were gathered around Papyrus.

"Boss!" Sans ran over to his brother, genuine panic in his eyelights. Papyrus looked like he'd been through hell, and with one bright blue spear through his chestplate, it was obvious who did it. His humerus was broken again with the rest of his arm lying on the snow beside him, and he was laying against the Snowdin shop with a tired scowl as blood seeped from the hole in his armor. Hoppi was busy pushing a cinabun into his open jaw, coaxing him into eating it. Dogaressa was grabbing a blanket to drape over him while Dogamy held their puppies. The other monsters were mumbling worriedly amongst themselves, offering other healing items and cushions to aid his recovery, as well as some encouraging words. If his brother wasn't so hurt, it would've been rather touching seeing so many monsters showing pure kindness.

"That fucking fish," Sans hissed under his breath, ready to go feral on her right that instant. But before he could, he saw Flowey pop out of the ground. "Not the time, flower." It was the only warning he'd get. The only reason he wasn't smited already was because Sans needed all the magic he had to take Undyne on.

"Undyne took (Y/N)!" Flowey cried, scared tears slipping down his petals. Sans felt his heart drop. He wasn't stupid, he could figure out why Undyne attacked Papyrus: to get to (Y/N).

"Heal Papyrus. I'll go save (Y/N)." Sans pointed at the flower, and for once, Flowey seemed willing to set aside their differences to help. If (Y/N) could look past their deadly nature to their good souls, Flowey owed it to her to try.

"Undyne took her to Alphys," Flowey said before disappearing into the ground to move closer to Papyrus. Sans nodded, the fury making his right red eye glow and smoke.

"Don't let him dust," he told the other monsters coldly, and they all nodded. They weren't planning on it. Then Sans teleported to the Royal Scientist's lab.

It was as dark and foreboding as any evil scientist's lair. Sans conjured a jagged bone to hold like a knife as he slipped inside, keeping his eyesockets open. He typically avoided Undyne and Alphys as much as he could, but since they involved Papyrus and (Y/N), they were going to pay with their lives.

It was obvious where they were since it was the only part of Alphys' lab that was lit up. At the back of the room was a gray machine with a bunch of dials and wires and one big screen in the middle that showed the progress. (Y/N) was hooked up to a bunch of wires, keeping her suspended in the air. Her red soul was in front of her, the color steadily draining from it like the machine was sucking the determination out of her. She looked pale—both from her soul being drained and her freshly fixed but now once again broken leg. The bone was shattered, and blood was dripping down her leg steadily. Her white lab coat was used to wrap the wound to stop her from bleeding out before they finished draining her soul. There was something symbolic about the proof of her purity being the reason she was still alive and the reason she was able to be saved. But that was looking too far into it.

"Why do you fuckers need determination?" (Y/N) was asking, her (S/C) skin ghostly but the fight still in her. Even in death, she was using his dialect. That's my girl. Sans found himself smirking proudly.

"So I can replicate human souls so we can destroy the barrier," Alphys said coldly. She was a yellow dinosaur with a filthy lab coat—a complete contrast to (Y/N)'s—and high waisted black skirt. The glasses on her nose did little to hide the crazy in her eyes. Her tone hardly ever seemed to lift out of cold and unfeeling, and when it did, it was only to express her pure craziness.