I will be alternating between your POV and Sans POV this chapter

First you tried to figure out what each symbol meant. Some did nothing, some burned whatever item you'd carved the symbol into and gave a bit of magic to, some all but exploded (which was why you started wearing a mask that you'd taken from the lab, one of those welding masks at that), some seemed to hold your magic and some actually seemed to expand on the magic before it was all used up. All in all promising. If you could somehow combine them to make one self sustaining rune and imprint on it, you could fix the C.O.R.E. The trouble was finding the right ones to make this sign, this imprint. That was hard to do without Sans help. Unfortunately for you, he was off hunting, you suspected that he was mad at you for some reason. You just couldn't leave him to die here though or slip back into insanity... he was barely holding onto what little sanity he did have. You turned back to your work, a crap ton of notes on your observations, different types of runes, and different types of combinations you could try, plus a crap ton of sticks you'd collected as test subjects.

❄️❄️❄️❄️❄️❄️

Sans puffed out a breath in annoyance. Aliza died. She died, she reset. So now instead of helping [Y/N] with her project like he wanted, he was hunting the brat down. Again. Sure it was frustrating how insistent [Y/N] was being on this, but he supposed it was a good thing. He'd gotten used to this way of life, he lost hope easily, so when things seemed impossible, he gave up quickly. Except this brat. Aliza was the exception. If Aliza hadn't died, he probably would have went back to help [Y/N] with a bit of food as a peace offering. Maybe. He would go back, he just had to locate them first and recapture them. With his newfound knowledge, he also knew some spots that he couldn't remember before for a little brat like her to hide undetected. Sans stopped at a certain igloo in the woods, tempted to just shoot bones inside and drag her out, but he needed her alive, not dead. So he reached inside, catching fabric in-between his phalanges.

❄️❄️❄️❄️❄️❄️

So... there was a few (lot) of failures. The house nearly caught fire three times. "WHAT ARE YOU DOING?" Papyrus had asked upon smelling smoke. "Working on something to fix the C.O.R.E. It's trail and error at this point." You replied as you carved the next imprint and transferred a small amount of magic into it. The mark glowed [M/C] a moment before fading, the magic spread along the length of the stick and held for a moment before the stick burst apart in sparks of magic and wood. You sighed. "More error at the moment, but I'll figure it out." Papyrus patted your shoulder. "YOU WILL, I BELIEVE IN YOU!" He cheered. But it only made you sigh again. "HUMAN?"
"I wish your brother had the same confidence in me that you did." That made Papyrus sigh as well. "SANS... ISN'T GOOD AT KEEPING HOPE. HE'S BEEN THROUGH A LOT, SO I DON'T PARTICULARLY BLAME HIM EITHER. BUT YOU WILL PROVE TO HIM THAT IT STILL EXISTS! YOU ARE THE CHILD OF SNOWDIN AFTER ALL!" You gave him a small smile. "Thanks Pap, the pep talk was just what I needed."
"NYEH HEH HEH, YOU'RE WELCOME!"

❄️❄️❄️❄️❄️❄️

"L-let me go! P-please! Sans!" Aliza's pleas fell on deaf ears as Sans dragged her to his home. "How did you know about the igloo?!" He growled and covered her mouth. " s h u t u p . " He hissed. He grabbed her again and started pulling her along. "Is she still alive..? The other human?" Sans growled again. "... yes.." He hated that Aliza was talking about her. He was once again tempted to just kill her then and there. "Why? What is the difference between me and her? Why must I die and she gets to live?" He paused in his stride, turning to stare the small, trembling girl down. "she's not an annoying brat for one. she's fixing the c.o.r.e for two. that makes her much more valuable alive than you." There was a third reason, but there was no way he was telling her that. "She's... fixing the C.O.R.E? How?!" Sans snarled. " n o n e o f y o u r b u s i n e s s . " It's not like it would work anyway.. he continued pulling her along.

❄️❄️❄️❄️❄️❄️

You stared at the stick. It was on fire... but it wasn't burning the stick. You.... you had done it. You'd made the imprint! You just had to carve this on the C.O.R.E at a much larger scale and food, power, magic, everything would be back! Waterfall would be lit up again, the farms would be growing crops again, the weather would stabilize, and monsters would hopefully normalize. That was your hope anyway. Grillby wouldn't be in constant pain anymore, Papyrus wouldn't have to eat humans anymore, Sans could stop starving himself! Maybe Undyne would stop trying to kill you- you actually doubted that. That was more of a personal grudge against Sans.. either way, now wasn't the time to think about that. You grabbed your fire stick and charged down the stairs and out the door. You still had a lot to learn about this new creation of yours, but you could at least show Sans there was hope yet! Your feet carried you to the shed where Sans was throwing a familiar little girl in a cage. "I DID IT!!!" He froze, 'eye' shrinking slightly at the flaming stick. "A way to fix the C.O.R.E." You handed it to him as he stared at it in wonder. "[Y/N].." What happened next, was the last thing you were expecting. He kissed you.