This idea would not die once I saw it in a vine. So here it is, out of my head. Enjoy.
Undertale: Revenge is a Dish Best Served with Butter
"FOR THE LAST TIME, SANS, STOP PLAGUING MY LIFE WITH INCIDENTAL MUSIC!"
The trombone gave one last trumpet before Sans disappeared back into his room, snickers trailing down the stairs to grate against Papyrus' skull. Gritting his teeth, the taller skeleton returned to his cooking. He had a surprise spaghetti concoction to show off to his brother tonight and all the puns and jokes and incidental music in the world wasn't going to ruin it!
…Though Sans had been particularly punny today. It was getting on Papyrus' nerves.
Maybe some well-deserved revenge was in order?
Papyrus grinned rather comically before putting his cooking on hold. Slinking upstairs as quietly as his battle body allowed, the taller skeleton went into his room and turned on his computer. He resisted the urge to check his social media messages. The last thing he needed was to see that anonymous user harassing him with witty puns.
He was pretty sure whoever this was had to know Sans. Lots of those jokes sounded like things the shorter skeleton would say to him. Sans denied having any involvement with such a miscreant though, so Papyrus let the case rest. Besides, Sans was rather good at comforting him after one of his meltdowns over said anonymous messages.
Heading onto the internet, he hunted for revenge ideas. Of course, it should be cooking based. He was good at that. Preferably with spaghetti but, with the level of jokester Sans had hit today, the taller sibling was willing to go with anything. He skimmed the results, mainly videos.
"AHA! THE GREAT PAPYRUS CAN DO THIS! NYEH HEH HEH!"
Slinking back down the stairs, Papyrus tossed on his apron and carefully pulled off his gloves. It wouldn't do to ruin any part of his battle body in his revenge scheme. Digging in the hot fridge, he pulled out the leftovers from Grillby's that Sans had brought home yesterday. He put these on the table, along with a full bottle of ketchup.
Then he grabbed the butter from the cupboard and got to work. Revenge would be oily…and fatty…and not at all sweet, but Papyrus was going with it anyways!
.o.o.o.o.
"SANS! GET DOWN HERE! YOU HAVEN'T EATEN LUNCH YET! YOUR LEFTOVERS ARE DOWN HERE ON THE TABLE FOR YOU!"
Papyrus quickly realized a flaw in his plan.
"AND NO TELEPORTING! OR SHORTCUTS! USE THE STAIRS LIKE A NORMAL MONSTER WOULD!"
"aw, but pa~p!" Sans called from upstairs.
"NOW! BEFORE IT GETS COLD AND GROSS!"
A dramatic sigh came from the shorter skeleton's room before the door opened. The older skeleton trudged down the stairs, hands shoved in the pockets of his jacket, smile dampened a bit. He looked a little tired.
"I'VE GOT FRESH KETCHUP~!" Papyrus sang as an added motivator.
Previously slow, tired steps became quick and frantic. Sans certainly loved his ketchup. Papyrus tried not to grin as his sibling whipped around the corner and charged into the kitchen.
And promptly wiped out on the butter-coated floor.
"NYEH HEH HEH! VICTORY FOR THE GREAT PAPYRUS!" he crowed merrily.
Sans groaned from where he lay sprawled in the middle of the kitchen. His slippers had flown off his feet in the fall and the back of his shorts and coat were streaked with butter. The skeleton groaned, raising one hand to rub his head. He rolled onto his hands and knees, coughing.
"ow… pap, did you do this?"
"YES I DID! THAT IS REVENGE FOR ALL THE PUNS AND JOKING AND TROMBONING—DON'T YOU DARE PUN THAT—AND GENERAL LAZINESS YOU HAVE DISPLAYED TODAY. NOW THAT THAT IS SETTLED, YOU MAY HAVE YOUR LEFTOVERS. I WASN'T LYING ABOUT THE KETCHUP. I REALLY DID GET A NEW BOT—"
"pap?"
"YES?" Papyrus snapped to attention. Sans sounded…odd. "ARE YOU ALL RIGHT?"
"uh… i think i hit my head a lot harder than you meant me too, bro."
Papyrus froze. Images of cracked skulls and broken bones flashed in his mind. He zipped to his sibling's side instantly, half-hauling Sans into his lap, checking him over.
He had just wanted to get back at his brother, not actually hurt him! That wasn't part of the plan at all! He wanted to cry suddenly. He was such a horrible younger brother!
"uh, bro? bro? bro!"
"WHERE DOES IT HURT? I DON'T SEE ANYTHING BROKEN, SANS!"
"bro, relax. it's nothing major, honest. come on, look me in the face."
Papyrus forced himself to breathe and settle down. Overall, it looked like every bone in his brother's body was relatively unscathed. He shifted, looking at his older brother in relief. Sans was probably just a little rattled. No harm, no foul.
…
"SANS, WHAT ARE YOU SHOWING ME?"
Something long and white was held between two of Sans' fingers. Papyrus frowned, squinting at it. It looked like a bone…but nothing on Sans was broken. He'd seen to that himself with his check-up, frantic as it was. He couldn't puzzle out where this strange object could have come from.
"my tooth. it came out when i fell."
Papyrus' jaw dropped at the same moment his sockets locked onto Sans' smile. There was a dark gap in it now. How had he not seen it? It was so obvious considering how much his older brother smiled!
"…UGAAAAAAAAH! WHAT HAVE I DONE? BROTHER, I'VE RUINED YOUR SMILE WITH MY PRANK! I'M A HORRIBLE BROTHER! I'M SO SO SO SORRRRYYYYYYYY!"
"bro, cool it. it's fine, really. it was my last baby tooth, anyways. it was bound to come out one of these days," Sans reassured, hugging Papyrus to try and cease his sibling's bawling.
"R-R-REALLY?" Papyrus sobbed, the waterworks shutting off briefly.
"really," Sans nodded. "besides, bro, now i look even better."
"YOU DO?"
"yeah," Sans grinned widely. "don't i look…beau-tooth-ful?"
"…"
Papyrus shoved Sans off of his lap, stood up, and brushed the butter and dust from his apron and legs.
"BROTHER."
"yeah?"
"THAT WAS THE WORST PUN YOU'VE SAID ALL DAY!"
"you've been keeping track of those? wow, bro. i didn't know you were so dedicated."
Papyrus screamed in frustration and stomped to his room. Sans laughed briefly before pocketing the tooth and taking his food back to his room. He absently stuck the tooth under his pillow, mostly because he had nowhere else to put it. He hadn't really paid attention when the human kid had told him about the tooth fairy.
He was rather confused when he found a few gold coins there the next morning.
Maybe he'd better start paying better attention to the kid's human fairytales from now on.