It was a normal day on the surface... Well, as normal as it could be for a monster. While a majority of the humans accepted the monsters, more or less, there was a large minority that had a greater voice than those that accepted them. Despite living in the city at the bottom of Mount Ebott with the humans, the neighborhood was split in half; monsters living on one side, humans on the other. Most didn't try to cross this new barrier, even with Frisk's and Asgore's attempts to help bring the two races together. Some monsters remained Underground, deciding staying there was better than living aboveground, where monsters lacked any citizenship, and-in their minds-was safer.
Despite all these problems facing them, Papyrus took Frisk's side of thinking that there could be peace between the two species. He remained happy; after all, he had everything he needed. He was surrounded by his friends in the neighborhood, he and Sans were living together in an apartment (and it was almost like they were still Underground, Papyrus was doing all the cleaning, and Sans was being his lazy self), and Frisk came over whenever they weren't too busy to have a sleepover. Papyrus's cooking had even improved quite a bit since he started cooking classes!
One of Papyrus's favorite thing to do was to travel to the top of the mountain with Sans to see the stars, seeing as they didn't get a very good view of them in the city. They did it once a week, camping up on the summit to look at stars as Sans pointed out several constellations, or they tried to see who could find a shooting star first (Papyrus was always the one who spotted it first!). Sometimes they could even get the others to tag along, when they weren't very busy. Undyne was still leader of the Royal Guard, seeing as the slight hostilities between humans and monsters meant that the Guard couldn't be disbanded completely, Alphys was working with scientists that didn't mind working with monsters, Mettaton was still a star in the monster community although he was slowly gaining popularity with the humans, and the Dreemurrs (Frisk included) worked on settling relations between monsters and humans.
Papyrus was leading the way up the path as the sun finished setting, Sans slowly climbing behind him. "COME ON, YOU LAYBONES! WE'RE ALMOST TO THE TOP!"
"i'm... i'm comin' pap..." Sans replied between huffs, staring down at the ground.
"WELL, HURRY UP! YOU PROMISED TO SHOW ME MORE CONSTELLATIONS TONIGHT!"
Sans slowed, dropping to his knees. "... ya... ya know what, pap? go on without me..."
"WHAT? WHY SHOULD I?" Papyrus asked, glancing back at his brother. "WE'RE ALMOST THERE, YOU SHOULD BE ABLE TO MAKE IT!"
"i'm... i'm not gonna make it, paps..." Sans repeated, lying down on his side and closing his eyesockets.
"SANS..." The smaller skeleton didn't move, and Papyrus sighed. "ALRIGHT... I'LL CARRY YOU."
"nah... don't worry, paps... it's okay, you've been doin' it too long now..."
"I'M NOT GONNA JUST LEAVE YOU HERE, SANS! BESIDES, YOU PROMISED TO TEACH ME MORE ABOUT THE CONSTELLATIONS!"
"heh... sorry paps... i'm no good with promises, ya know that."
Papyrus knelt down besides Sans, scooping the smaller skeleton into his arms. "COME ON, SANS. WE'LL MAKE IT TO THE TOP TOGETHER."
"h-heh... l-love ya, pap..."
"I LOVE YOU TOO, SANS."
It was nearing midnight, and Papyrus was sitting leaning his back against a rock. Sans was on his lap, his eyesockets closed. "ARE THOSE STARS PART OF A CONSTELLATION, BROTHER?" Papyrus asked, trying to wake up his brother. The smaller skeleton made no response. "SANS? SANS, WAKE UP!" No response. "... PLEASE?" When Sans still didn't say anything, Papyrus looked down at him in concern. "S-SANS?" He received no response and the skeleton jumped to his feet, clinging Sans tightly to his chest. A white heart, cracks expanding from the center of it, had appeared on Sans's chest. "I-I'M GOING TO B-BRING YOU TO ALPHYS, BROTHER. M-MAYBE YOU'RE SICK." He turned, racing down the mountain with Sans tucked tightly to him. He ran the entire way to the house Alphys and Undyne shared, shifting Sans to one arm as he banged on the door.
"WHO THE HELL IS MAKING THAT RACKET!" Undyne shouted from inside when she woke up, the angry footsteps being heard as he raced down the hall. She opened the door, her eyes widening in shock when she saw Papyrus holding Sans. "P-Papyrus? What-"
"S-SANS IS SICK! I N-NEED ALPHYS TO LOOK AT HIM IMMEDIATELY!"
"U-U-Undyne? Wh-Who is..." Alphys looked around Undyne, freezing when she saw the two. "O-Oh..."
"ALPHYS, PLEASE! YOU NEED TO HELP HIM!"
"C-Come set h-h-him on the couch." Alphys says, and Undyne stepped out of the way before Papyrus could run her over as he ran in. Instead of setting Sans on the couch, he held tightly onto his brother; placing him on his lap as Papyrus sat on the couch.
"IT'S ALRIGHT, SANS. ALPHYS WILL MAKE YOU FEEL BETTER!" Papyrus said to him, not noticing a knowing glance between Undyne and Alphys.
"P-P-Papyrus, I-I..." Alphys started, though she broke off quickly. Her and Undyne's eyes widened in horror, watching the cracks in Sans's Soul expand.
"S-SANS! P-PLEASE, WAKE UP! Y-YOU'RE... Y-YOU'RE..." Papyrus placed his hand over Sans's Soul, closing his eyes as he focused his magic, trying to heal him. "S-SANS, PLEASE!" Papyrus's magic did nothing, the Soul cracking even further. As he hugged his brother tightly, Papyrus opened his eyesockets in time to watch Sans's Soul shatter. Papyrus lunged off the couch, clinging Sans tighter. "A-ALPHYS! S-SANS, P-PLEASE!" Papyrus fell to his knees, watching Sans begin to crumble to dust in his arms. "SAAAAAANNSSSS!"
Alphys grabbed Undyne's hand, squeezing it tightly as the two watched Papyrus begin to sob, clinging to his brother the best he could.
The most painful goodbyes are the ones that are never said, and never explained.