(Author's Note: This is a one-shot first off. And secondly, while I have delved into the lore and characters of Undertale, I have not actually played it myself. Despite all that, this is my AU of sorts, and in it, things are somewhat different to normal Undertale. Also, I know that other writers have probably already explored the ideas I'm putting in this fanfic, but I have an urge to write and Undertale fic and I am doing so regardless of other written works that may have already tread here. All that aside however, I hope you enjoy my little twist to Gaster and the Players. And I hope it makes some sense, as I finished this at about 4:05 AM.)

The void.

A lifeless place.

Or so it would seem anyway.

For within this infinite expanse of gray nothingness resides one man, though whether he can be considered such any longer is... debatable.

But such is the life of former Royal Scientist W.D. Gaster. Even when he was still in the world and not scattered across all of space-time, many things he sought were debatable at best, and improbable at worst. Such as the CORE. An endless source of self perpetuating magical energy that would power all of the Underground. Many of his peers at the time called him mad. Said his idea would never amount to anything. They used a lack of resources against him first, claiming that they couldn't even build a network robust enough to harness that much power. But when he proved them wrong by doing just that, they changed their tune. Rather than saying the proper framework couldn't be built, they said that they'd never be able to convert and distribute that much arcane energy effectively. Yet, against all odds, he had. Proving himself not only capable of doing what he'd said he would, but also proving himself worthy of the title given to him by King Asgore as well.

Despite all of that work, all that study, research, and defense against the vultures that were his colleagues, he still hadn't been satisfied. He'd wanted to do more, to know more.

That had been his downfall.

Not content to sit back and do nothing after discovering a way to provide the Underground with an endless source of usable energy, he began to delve into the nature of the universe itself. More specifically the nature of time and how it truly functioned. For years he'd wondered if every choice a person did or didn't make created an alternate reality. A branching path in existence where things ended up differently as a result. He'd also wondered if it were possible to go back. To change what had already been done.

To undo past mistakes.

So he dug. His world eventually becoming a never-ending maelstrom of theories, equations, and problems. One's which seemed to have no definitive solution.

Until he delved into the ancient records of the Human-Monster War and found... it.

DETERMINATION.

The force which all humans and monsters held within themselves.

A force that, when used properly, could change reality itself.

But, as always, there was a problem. His kind, monsters, didn't produce enough DETERMINATION on their own to be able to persist after death as humans did, let alone enough to travel back through time and change the past. So he began searching for ways to get more. And, as a result, built the DT Extractor. An alien looking device to be sure, but one that suited his purposes perfectly. He started out small at first, channeling small amounts of DETERMINATION through the DT Extractor and injecting it into various subjects he had... "acquired" from different parts of the Underground, but once the tests were done he found that the subjects bodies were able to handle the extra amounts of DETERMINATION he'd placed within them.

Deciding that it was safe to continue, and wanting to find a definite cut off point for how much DETERMINATION a monster needed to have in order to affect reality around itself, he gradually increased the amounts he'd been injecting. But, much to his dismay, the increased amounts of DETERMINATION caused the subjects bodies to melt and even fuse together into new, horrific forms. Almost as if they were unable to handle too much of it at one time. So, somewhat discouraged by this outcome, he quietly disposed of the subjects and took some time off from the experiment to think.

Eventually he came up with an idea, albeit a very risky one.

If the creatures he'd used were unable to complete his goal, then he would do it himself. His reasoning was that they simply lacked enough willpower to hold themselves together effectively. So, knowing his own limits, he figured he should have no problem utilizing the DETERMINATION from the Extractor to complete the experiment.

But he was no fool of course. Not wanting his latest project to be completely forgotten should things go awry, he left meticulously detailed notes on his most recent discoveries, save for the subjects he'd used, within his lab so his more trusted colleagues could learn from and hopefully use them to finish his work one day.

BUT THAT NEVER HAPPENED. DID IT, GASTER?

"No... it did not." A semi-melted figure clad in a dark coat, pants, shoes, and sweater replied, it's cracked porcelain-like face sagging slightly.

THE DETERMINATION WAS TOO MUCH FOR YOUR BODY TO HANDLE. AND SO, IN YOUR DESPERATION, YOU SOUGHT OUT YOUR CREATION AND FELL INTO IT, HOPING THE MAGICAL ENERGIES RAGING AT IT'S HEART WOULD HALT THE PROCESS. STABILIZE IT.

"But they didn't." He said softly, his gaze falling.

NO, THEY DIDN'T. INSTEAD THE MAGIC, COMBINED WITH THE PURE DETERMINATION COURSING THROUGH YOUR RAPIDLY DETERIORATING FORM, AND YOUR DESIRE TO CHANGE THE PAST SHATTERED YOU, IN BODY AND MIND, SCATTERING YOUR VERY BEING THROUGHOUT ALL OF TIME AND SPACE. HAD IT NOT BEEN FOR OUR INTERVENTION, YOU WOULD STILL BE OUT THERE DRIFTING FROM WORLD TO WORLD, TIMELINE TO TIMELINE. COMPLETELY UNAWARE OF EVERYTHING. EVEN YOURSELF.

"I-I know." Gaster replied, flinching a bit as he cast a glance at the windows in space-for lack of a better word-floating before him. The foremost of which showed a small child in a blue sweater with two pink stripes blazoned across its surface, a pair of shorts, and black boots walking through a seemingly deserted Snowdin, the knife they held as visible as the crazed smile etched across their face. Gaster sighed. It wasn't an unfamiliar sight. As a matter of fact, it's something he'd seen many, many times before.

At this point, however, he'd given up hope on seeing anyone in the Underground live the moment they'd reset everything after the "Pacifist Route" as they called it. It wasn't as though watching the small child, whom they referred to as Frisk, murder everyone he'd ever known still brought him discomfort though. Rather, it was the thought that one of them was guiding Frisks actions that bothered him. That one of them had made the conscious choice to kill everyone he'd ever known or cared about simply for curiosities sake...

That's what truly terrified him. The sheer callous disregard for life these beings currently held, when only years before they had done all they could to ensure everyone in the Underground had the happiest ending possible, disturbed him deeply.

He supposed that, in a way, he hadn't been so different from them himself back when he was still in the world, going about his tests and experiments. Not caring how it affected those he used...

BUT, FORTUNATELY FOR YOU GASTER...

It went on, blatantly ignoring his lack of response, physical or otherwise.

WE DECIDED TO BE MERCIFUL ON THAT DAY.

AND YOU KNOW WHAT THAT MEANS, OLD FRIEND?

Gaster nodded slightly, having returned from his brief reverie.

He did know. All too well. But that wouldn't stop it from reminding him. It never did.

IT MEANS THAT YOU ONLY EXIST BECAUSE WE ALLOW IT. IT IS BECAUSE OF OUR MERCY THAT YOU STILL THINK AND MOVE.

NEVER FORGET THAT.

"I won't." He said quietly as he felt it give off a wave of sickeningly smug satisfaction, before slowly retreating, leaving him standing there in the midst of the colorless void.

Alone.

He sighed, turning his attention back to the windows once more. All of which showed him infinite reflections of the world he once called home. A place once so vibrant and filled with life, now reduced to nothing more than an endless series of repeating simulations. A place that he wished, despite how pointless it might be, he could return to.

But, of course, he couldn't.

For there was no escape from this place for him. No hope.

This was the price he was to pay for his foolishness.

To stand watch over the timelines for all eternity, unable to intervene, as his world was reset over and over again for the amusement of beings beyond his comprehension.

With this thought still lingering in his mind, Gaster buried his ruined face in his hands...

And wept.