[WARNING: This story may contain spoilers, if you don't want details of the game ruined for you than I would recommend not reading this until after you've finished playing Undertale.]
"Don't you have anything better to do?"
These were the final words Asriel spoke to Frisk as they met for the last time in the ruins, among the bed of yellow buttercups he once tended to long ago. The white furred, goat-like monster child stood among the flowers, watering them with a tin watering can and silently crying while Frisk, a human child he had recently befriended, slowly walked away, turning back once more at the entrance of the ruins before leaving in defeat. Tears dripped from his eyes and onto his green and yellow striped sweater while he hummed to himself in an attempt to hide the fact that he truly, deeply wanted to go with Frisk and see his mother and father again, to make friends, go back to school and finally be loved and appreciated again. To him though this was a fantasy, the reality was even if he had his old soul back, even if he had the ability to go back to his parents he couldn't bring himself to do it, knowing everything he had done before and after he met Frisk.
"What an IDIOT!" A familiar shrill voice said among the buttercups as one rose from the ground, possessing a deceitfully innocent looking smiling face.
"Oh boy! I can't WAIT to see the look on his face when you know who takes control and guts each of his precious friends like a fish! Toriel, Asgore, Papyrus, Smiley Trashbag, they're all in for a BIG surprise veeery soon." The flower said teasingly, laughing to himself before his face became distorted, eyes turning black and a jagged smile forming across his face.
"And YOU made it all possible Asriel! Frisk, Chara, everyone you know, everyone you love will suffer and die in the end, including us! Hahahaha! And it's all your fault." Flowey laughed, causing Asriel to lose his composure and fall to his knees, sobbing into his hands bitterly.
"I know! I know! I'm sorry..." Asriel lamented, his voice shaking. Flowey's eyes turned red, his pupils turned green and he suddenly began to transform, turning massive, big enough to fill the entire cave. He turned into a creature with a bald goat-like skull, numerous wiggling eyes and massive arms made of vines which grabbed Asriel by his little arms and lifted him in the air as if to tear him apart.
"You're not sorry enough! You'll NEVER be sorry enough Asriel! And pretty soon you'll be the person you were always meant to be... Me! It's too late for you now Asriel, the old you is just a little helpless morsel while I, Flowey, reign supreme! Now I'll finally get rid of you once and for all, and you'll become Flowey forever!" Flowey said darkly, the massive goat skull opening, laughing maniacally as Asriel was slowly brought struggling towards the gaping maw.
"No! Stop! Don't eat me!" Asriel begged as he was brought towards Flowey. Shutting his eyes tightly, he prepared for the end.
"Mom! Dad! Frisk! Somebody help!" Asriel screamed in terror as vines began wrapping around his arms and body.
And somebody came...
Suddenly, a barrage of blue, glowing bone shaped spears pierced Flowey's body, forcing him to scream in agony and release Asriel, the child falling to the ground with a thud. Flowey's body trembled and oozed crimson, coagulated blood. He scanned his eyes around the cave for his attacker.
"Ah! What!?... YOU!" Flowey said, catching sight of the one who hurt him. The room fell silent save for a pair of shoes clicking across the stone floor as the attacker approached, a man in a black trench coat with a wide brimmed black fedora. He was skeletal in appearance and his face consisted of hollow eye sockets and a wide smile with numerous cracks across his skull. The figure removed his hat and tossed it aside, red lights appearing in his eye sockets which burned like red suns.
"You think this changes anything old man!? You think you could stop this!?" Flowey screamed angrily before another barrage of bone spears impaled his enormous body, red blood spraying all over the flowers below as he thrashed around like an injured giant.
"Silence." The skeletal man said coldly.
"What a wicked creature you are, attacking a child. Surely you must be one of Dr. Alphys' hackjobs, I swear that woman has no discipline." The man in black said in a seemingly emotionless, analytical tone. Asriel was shocked, looking at the mysterious man as if he was seeing a ghost.
"G-Gaster!?" Asriel said in confusion, Gaster did not respond to Asriel save for glancing at him momentarily before returning his attention to the heavily damaged Flowey, who was lurching towards him.
"YOU! I'LL RIP YOU APA-" Flowey started to say before Gaster raised his hand and slammed him into the floor with gravity based magic, crushing him against the ground as if he were being squeezed by a massive vice. Flowey struggled and screamed as he, despite being in such a massive and powerful form, found himself helpless before the man sized skeleton and his powerful magic.
"Clearly you don't understand the "gravity" of your situation." Gaster said. "Your size and mass make you especially vulnerable to my gravity based magic. Simply put, you cannot win."
As Gaster said this he increased the power of his attack, crushing Flowey even more as Asriel ran behind Gaster for protection. The massive gravity spell was focused entirely on Flowey, so nothing else was being drawn towards him, it was similar to the gravity attacks his son sans used, allowing him to lift objects from a distance at will and toss them about effortlessly.
"I always find myself cleaning up Alphys' mess. Nevertheless, despite how disgusting you are, I find you fascinating, this experience has certainly been educational. It's a shame I no longer have a lab to study you in." Gaster said as he gestured his left hand, summoning a trio of floating machines that resembled dinosaur skulls.
"Farewell." Gaster said as the skulls opened their mouths and charged energy into them and fired three massive beams which engulfed the screaming Flowey and caused a massive explosion in the room and a cave in on the other side. Asriel shielded himself from the falling debris while Gaster stood motionless, gesturing his Gaster Blasters to disappear. Everything was quiet and when the dust settled Flowey was nowhere to be seen, only a massive pile of debris, blood and destruction.
"So he managed to escape." Gaster said to himself, Asriel tugged on the back of his coat.
"T-Thank you doctor. I thought I was a goner. But..." Asriel said, confused by what just happened. "Aren't you... y'know."
"Dead?" Gaster asked, completing Asriel's sentence. "Yes, and sadly so are you. You are currently within the void, the space between life and death and my abode for all eternity." Gaster explained as he spread his arms to emphasize the world around him. "I've been watching the universe from this vantage point for a long time, I've seen the same events relive themselves again and again. Unlike me you will fade from existence soon enough without a soul to anchor you to reality, and from that point Flowey will return to living world to terrorize your friends once again."
Asriel slowly lowered his head, knowing that he is ultimately responsible for everything that is happening.
"However, that is not the most pressing issue at hand. This timeline and all timelines in fact are doomed and cannot be corrected as things stand now." Gaster said, an imperative air to his voice. "Timelines are splitting, amalgamating, coming into existence and disappearing again and again so fast reality cannot keep up with the changes. I've warned my son about this, the universe cannot handle these kinds of fluctuations, soon enough it will unravel at the seams and all existence will be destroyed in one big crunch."
Asriel took in the insanity he was being told by Gaster and began to panic, he gasped upon hearing about the destruction of the universe.
"The big crunch!?" Asriel said in a panicked voice.
"Indeed. When that happens time itself will compress to a singularity, ending all things in an instant." Dr. Gaster said, lowering his head.
"How do we stop that! There has to be a way!" Asriel begged, clutching Gaster's trench coat. Gaster smiled.
"Fortunately there is. You may be the last hope this universe has to survive Asriel Dreemurr. In this time all paths lead to destruction, your old friend Chara has possession of Frisk and is in turn controlled by a much greater, darker force. I cannot leave the void and help you, but from here I can use my power to help you perform the Ultimate Reset." Gaster said.
"Ultimate Reset?" Asriel asked. "Gaster, I can't do that. Frisk worked so hard for everyone to be happy, to finally get the happy ending he deserves, that everyone deserves and-."
"There will be no happy ending Asriel." Gaster interrupted sternly. "Frisk's soul belongs to them now, just like Chara before him. Frisk's friends, the monsters and humanity as a whole will learn happiness only to be destroyed by their own savior, and in the end this timeline will cease to be as well, further expediting the big crunch." Dr. Gaster said, he knew that Frisk's soul was no longer his own, he had sold it to Chara long ago.
Asriel's heart sank upon hearing those words, he wanted Frisk of all people to be happy and knowing that Frisk was as doomed as he was, that he was sentenced to an unending cycle of death filled him with sorrow.
"I will send you back to a time before the timelines began to split, before Chara's death, before everything went wrong and THEY began to take over." Gaster said, placing his skeletal hands on Asriel's shoulders.
"Be warned, once you are sent back, you will lose the ability to reset, load or save. Such powers you have acquired from Chara after he merged with your soul after that day. If you die in the past, everything is doomed. You are the only one who can stop this, good luck little one." Gaster said. As he finished his sentence, his hands began to glow with crimson red energy which began to envelop Asriel. Asriel gasped as he found his powers increasing in strength, far more than he ever thought possible. Drying his tears one last time Asriel focused what was left of his fading soul power, he thought back to a time before everything went wrong, he thought back to the past when he and Chara were still alive. With this new power, the world went white, brighter than a thousand suns and time began to reverse itself.