[Undertale] Sans X Reader - Repeat, Eternally

Chapter 61 ~ Sans VS Gaster

I SOLVED MY PROBLEM I KNOW WHAT I'M DOING

You heard them before you saw them.

Outside of the hotel, Gaster and Sans  were in a fight-to-the-death battle.

Rushing outdoors, you see that Gaster wasn't wearing a labcoat. Instead, his coat was black and billowed out behind him as there were several hundred more Gaster Blasters than the 15 Sans had. His legs crossed one another at the ankles, and his skeletal palm was stretched outwards. In the hole of his hand, a ball of light alike to the ones in the Gaster Blasters' mouths was forming.

Gaster was extremely powerful compared to Sans. In his broken-down form, his power was suppressed and unattainable.... but when full and physical...

Gaster was literally unbeatable by any means, no matter who tried. 

There was a sharp pinch on your neck, and you slapped it. Only a mosquito.

You turned your attention towards the fight. There was no chance Sans would win this; he was already losing. Gaster could do this for years in a row. 

His energy was unbelievable. It was clear Sans was getting tired, but Gaster was just getting started. Sans was already serious while Gaster was intentionally missing his target in an attempt to deter or scare his son. 

To Gaster, an opponent like Sans was like Asgore when he first began training Undyne.

You weren't even sure Asgore would be able to match Gaster.

"Sans!" You yell, afraid. Fear shot through your veins as if it was your blood; he was doing this for you. He was facing certain death for you. Sooner or later, he'd get tired or Gaster would stop playing. "Please! You have to stop this!"

"Sorry, babe. Can't do that." He winked at you, his hands in his pockets. He gave a nonchalant shrug before nodding his Gaster Blasters towards Gaster. They fired as much as they could, but Gaster's Blasters were outnumbering Sans. "I gotta make up for the pain I caused ya." 

"Sans, you don't need to do this!" You look around, searching for something, anything, that could stop this. 

"I really think I gotta. I mean, I don't want to, and it's really tiring, but hey. Ya gotta do what ya gotta do." He shrugged again and whipped one skeletal hand out of his sweater, his eye lighting up with that familiar blue. Gaster flew into the air, and slammed into the ground, over and over.

He wasn't taking any damage at all.

You could see it.

Gaster's HP: 9999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999991

Sans HP: 1

Sans had done 8 damage so far. That was nothing to Gaster.

A glint caught your eye. You ran over to it, and realized...

it was a kitchen knife.

You turned to Gaster, who stood up.

Sans dodged another attack. He really couldn't dodge forever.

So you made a decision. 

You lunged for Gaster, knife in hand...

But someone jumped in front of Gaster.

"HUMAN....I'M SORRY, BUT I CANNOT CONDONE....VIOLENCE.... IN MY OWN FAMILY..."

The knife went right through their soul, as it split in half.

Your eyes widened and your heartbeat slowed as you realized what you had just done.