This will ultimately be a Gaster!Sans and Gaster!Papyrus AU story with my own twist to it.

Days seemed to blur into one long line of nothingness, when one didn't have the sun to go by as a reference. Monsters of every creed roamed the underground passageways numbly, mindlessly searching for ways to be productive before finally venturing out into the strange new lands that lay before them. After word came back about the different topographical regions, more Monsters ventured out, little by little, to make homes in this new world.

The Hotlands became the basis for an energy source, courtesy of the Core. Accessing this energy source required the scientists to live in that area, despite the almost-unbearable living conditions of Monsters not of a fiery persuasion. But efforts paid off and soon some comforts and luxuries began to develop, making life a little more bearable for all of Monster-kind.

Bearable did not mean 'great living situation'. It did not take away the fact that they were banished to this small region under the Earth. It did not take away the great war that killed off so many of their kind, many species being wiped out completely, and others being left the only of their kind.

It was a source of heavy sadness that the King and Queen remained the last of the Goat Monsters. Their Captain of the Royal Guard, Naiad, lost her husband in the war, and it wasn't until they were all underground that she found out she was pregnant with their egg clutch. The war had damaged her clutch and destroyed all but one egg, which she proclaimed was 'Undying', and would be named as such. That one miracle steeled her resolve to live on and be strong.

One other Monster last of his kind...was not so resolved.

Dr. W.D. Gaster had watched his entire species die out, and had been present at the merciless slaughter of his home village. It was a cruel knowledge that the Skeleton Monsters were not only very vulnerable to death due to their souls being the most unprotected, but that they were incredibly peaceful and not prone to taking up arms. Pacifists by nature, they were the very last Monster species to contribute, but by then, it was far too late.

Gaster knew why they were targeted despite their pacifism. Within humans themselves were skeletons, and thus Skeleton Monsters were considered a disgusting mockery of humankind.

He had hurried to his home village, in time to see it razed to the ground, dust choking the air. No one, from the elders to the baby bones, had been spared. His own childhood home was in flames, his family gone forever. In the midst of it all were the humans who destroyed it all, ready to move on to the next Skeleton Monster village, their weapons and clothing covered in dust, trophies from their kills saddled to their horses.

Gaster would have collapsed on the cold ground and welcomed death, had he not seen his younger sister's red scarf tied around the waist of one of the humans.

A fiery hate began to spark in his soul, his magic thrumming violently and demanding to be used. KILL THEM. KILL THE HUMANS. MURDEROUS BEASTS. The murderous beasts in question spotted him and drew their weapons, rushing forward. Gaster stood up and willed his magic forward, his intent toward them nothing but righteous retribution.

At his call, huge spike-like bones jutted from the ground, impaling every single one of those foul creatures, their blood staining the white snow red and splattering on his face and clothes, filling him with satisfaction, but also a morbid curiosity. How curious it was, that their wretched bodies remained, dripping fluid that their broken forms required to live. It was just as well. Their corpses would serve as a reminder, and as a warning.

Gaster walked up to the human that dared to take his sister's scarf, unwrapping it from around the man's waist, noting with grim satisfaction that the man was still alive. He took the human's face in his hand, tipping it upward to face him.

"I do hope your kill was worth it," he said, his eyes flickering with a blue and orange light. "Every human I find will suffer your fate." He tilted his head upward, seeing the full moon peek out from behind the clouds, illuminating his bones with an almost ethereal light.

"...It's a beautiful night..." he said softly, paying no heed to the human's ragged breathing. "...on nights like this...humans like you..."

He jerked his hands quickly, snapping the human's neck.

"...should be burning in Hell."

He let go of the human and tied his sister's scarf around his neck, taking in the scent of roses she so loved to grow throughout the year, turning and leaving the humans to suffer on their bone pikes, feeling the warmth of his soul cool until all that was left was ice.

The memories of that war stayed with him. The complete genocide of the Skeleton Monsters...the screams of his fellow Monsters as they ran for Mt. Ebott...the sight and scent of so much blood, the cries for mercy from humans he killed, both warriors and civilians...the darkness of the underground caverns...the visions never stopped, no matter how much time passed, every time he tried to sleep.

Thank the gods for Alphys, that sweet, young protege of his...had she not been around, there would have been no progress on anything. But he couldn't let her be in charge of EVERYTHING, no matter how enthusiastic she was.

It was still difficult. His lack of rest, short attention span, and tendency to lash out when stressed made for a very unproductive and unpleasant work experience. At the end of every day, he would pour over notes and think of a solution to his problem.

Over time, it became increasingly obvious that this wasn't an entirely fixable solution. Despite the experiments done to patch up his mind, it was clear that there were some things that couldn't be fixed. His mind was cracking and was sure to shatter, and then where would he be? What USE would he be? He couldn't let his broken mess of an existence be the legacy of the Skeleton Monsters.

If he couldn't FIX his mind...perhaps he could instead create a NEW one. A fresh start, unburdened by his past, clear and ready to bring solutions to the worst of the Underground's problems!

Not for the first time in his life, Gaster was grateful his primary area of research was Soul Study. It would be making a singular extraction of his soul that much easier to perform. On a day he set aside just for this, he calmed his mind as much as he could and girded his loins before summoning forth his soul.

How pitiful it looked, he thought to himself. So cold and shriveled, hardly worth the effort to extract a piece. Still, he had the incubation chamber all ready, and there was no going back now.

Honing his intent to have the soul piece extracted with his intellect and knowledge, he carefully sliced out a small sliver of his soul and immediately transferred it to the incubation chamber, watching with swimming vision, praying the soul piece would stabilize and remain formed. After a few long, painful minutes, the soul sliver stopped wavering and the stats remained stable.

He wasn't aware that he was laughing with relief until he was on the floor recovering, his hand clutched to his chest and trying to settle his soul down. His little soul sliver was going to be a new life to harvest his cleaned mind soon...he could rest now...just for a little while...