"Who the hell are you?"
"Oh, sorry buddy, forgot to introduce myself."
Dr. W. D. Gaster turned around and looked at the man behind him, shrouded in shadow. He couldn't quite make out the features on him.
"Could you step forward, please? Into the light, if you don't mind."
"Sure. No problem. By the way… you're the one who designed the CORE, right?"
"Yes, the CORE was my creation. But please, whatever it is that you have to say, say it now. I don't have time for small talk."
"Ok, makes sense, doc. I'll get right to the point."
The dark figure stepped into view. Gaster gasped in terror upon realizing who it was that he faced.
"right to the point. remember, gaster. you asked for it."
The scientist's face clenched into an ugly scowl as he raised his hands.
"Sans. What are you doing here?"
"don't joke around, gaster, you know why i'm here. there's a reason i named the little guys gasterblasters. i've got a bone to pick with you."
"You FOOL! You know full well your powers are nothing compared to mine!"
"then we'll test that, right? don't you want to find out if i've gotten any better?"
"I'll kill you, Sans. I won't hesitate."
"…"
"Fine. Beware the one who fights with hands. I am ready. Prepare yourself, Sans."
"ok, doc. fine. you want to make it like that, i'll make it like that. but remember. hands with holes aren't all that great for blocking."
"And 1 HP isn't that great for surviving. Anything. You were a fool to come here, Sans."
"and you were a fool for destroying all of my friends."
"D R."
Sans closed his eyes, opening them the second he pronounced the first syllable of "dirty," revealing a black depth that frightened even the seasoned Gaster.
"you ready to have a bad time?"
Gaster didn't even reply. Stepping forward, he raised his open palm. The hole in the middle of the hand began to slightly glow, before launching an invisible pulse of energy. Sans sidestepped, faster than the eye could see, and raised his hand, lifting jagged bones from the ground. Gaster rapidly dodged the attack, although his lab coat was slightly torn.
"Is that all that you can bring to the table, Sans?"
"you're jokes are even worse than mine. bonenie, here girl!"
Sans whistled loudly, as the animated skull of some massive creature flew into the room from… somewhere. Truth be told, even Sans didn't know where the gasterblasters went when they disappeared, but he really didn't care. They were strong, and they hated Gaster as much as he did. The second his punnily-named pet flew into the room and caught sight of Gaster, it began to growl, it's menacing sound filled with equal parts anger and hate toward Gaster and loyalty to Sans.
"sic em, girl."
Sans' pet didn't hesitate for a second. As it opened it's mouth, one of it's eyes glowed with blue flame, the telltale mark of the skeleton power used by it, the other gasterblasters, Sans, and his brother Papyrus. Opening its mouth, the gasterblaster let out a beam of blue energy that Gaster rather easily deflected with a backhanded blow.
"careful, bonenie, that guy's bad to the bone. but you know that, of course…"
Gaster saw Sans preparing another attack, and the furious gasterblaster inching toward him, white teeth barred. He felt like he was going to have a bad time. Sans suddenly disappeared, leaving behind an afterimage, appeared for a split-second in Gaster's face, and then disappeared again. He began purposefully leaving afterimages all over the place in order to psyche out Gaster.
It worked.
"Slow down, damn you!"
"How could anyone be that fast? He's not even a blur, he's simply too fast for the eye to track, even for me!"
"that's the funny thing, gaster. i've always been able to do this. you just never had the brains to stop underestimating me. you never had the brains to even consider that i could be a threat. well, gaster, i am. this is for all my friends. this is for my world. it's over, gaster."
Sans let out another shrill whistle, calling in several more gasterblasters, before flashing his hand over his eye, revealing a fiery blue pupil. He snapped his fingers, summoning in bones from the ground beneath his foe, forcing Gaster to leap into the air to avoid them. Sans then whipped out his hand, gripping Gaster's grey soul, turning it blue, and pushing forward, sending Gaster flying, until he was hanging in midair, seconds away from plummeting to his inevitable demise in the CORE. Sans snapped his fingers, and the gasterblasters fired, each one of them striking Gaster directly, until he was reduced to 1 HP. Sans then turned around and strolled off as Gaster fell into his creation, letting out a horrified scream as he fell.
"get dunked on."
As Sans left, Gaster was seeing his life flash before his eyes. He never knew what it was that he had done to incur Sans' wrath. He never even had a guess. But as he fell, he knew fear. The one who fought with hands now knew what it was like to be helpless. He never knew what would happen when he reached the reactor of the CORE.
But he was soon to find out. As he fell into the CORE's blinding light, he felt his body being rended apart, becoming immaterial. He felt himself fading out of existence. And he felt his SOUL being forcibly ripped from his body. Without a body to hold it, the SOUL mutated into a strange creature, a red, infantile monstrosity, still in an embryonic state. Meanwhile, even without a SOUL, his body somehow remained intact. He couldn't guess why. So he didn't try. He just left.
Gaster called out for his old associates, those he had personally hired.
But no one came. All he saw was a strange, unknown, yellow-skinned scientist, along with a robot built by her and her personal assistants.
God, he thought.
I'm gone.