Author's Note: This is an old fic, so the quality may not be the greatest. Also, this is based on the Friendswap AU, which is an AU where Papyrus is swapped with Undyne and Sans is swapped with Alphys. No one else is swapped.
The sounds of dripping water echoed against the stone walls of the tight corridor as a lone child walked down it, stepping into the occasional puddle of water. Despite wearing pink ballet shoes, they paid no mind to the mud collecting on them, dirtying what used to be a pretty pastel pink. They instead stared straight ahead listlessly as they walked on, turning a corner to see a rickety wooden bridge. The sight, along with the sound of the howling wind, filled them with determination.
After SAVing, they continued on––only to stop when they heard feet hitting against the wood behind them. They did not bother to turn as the footsteps stopped right behind them.
"Yo!" a voice called out. The human decided then to face the voice, making its owner––a monster child––flinch at the sight of their bright red eyes. "I-I-I don't know who you are," he began, "or why you did what you did. But!" He puffed out his chest, despite his trembling legs. "I know you h-hurt a lot of p-people." He put one foot forward. "And I-I c-can't let you do that a-anymore!"
The human tilted their head slightly at Monster Kid, eyes seemingly burrowing into his SOUL, before a huge, sinister grin invaded their face and they took a step closer.
Monster Kid's eyes widened as he winced back a step. "Yo, w-what's with that face?" they asked, their pupils contracting in fear.
With a bubble of laughter, the human rushed toward the monster. With a chkk-chkk-chkk, they both became monochrome, and the human's SOUL floated out of their chest. Without a moment's hesitation, they selected the FIGHT option, and––with a graceful spin––kicked at Monster Kid.
...Only to connect with metal instead. A sharp clang rang out as Papyrus stumbled backward, his breastplate now with a considerable oblique dent in it. With a grunt, he fell to one patella, panting heavily.
"P-Papyrus!" Monster Kid shouted, their lips trembling as they looked at him. "You're––You're hurt!"
Papyrus managed to let out a weak chuckle. "Hurt? It's... nothing," he said, struggling to speak with several cracked ribs. "Next time, run when I tell you to run."
"B-B-But––"
"I got this!" he interrupted, briefly sending a glare at Monster Kid. "Just go!" With a last desperate look at the human, Monster Kid ran away, Fleeing from the battle.
Papyrus let out a wheeze, his breaths labored. Somehow, however, a small smile found its way on his face. "Heh... 'Nothing.' With just one kick, I'm––" A series of coughs wracked his being suddenly. "D-Dammit," he cursed, not caring that the human––no, the abomination in front of him was a kid. They appeared unfazed by the word anyhow as they stared unemotionally at him. An involuntary convulsion coursed through him, sending some of his dust into the air; his legs were beginning to turn into dust.
"Undyne..." he whispered, closing his eye sockets to stop the burning tears he could feel coming. "Sans... Asgore... I've..." Another wheeze. "I've failed you all..." The child seemed to be satisfied by the sentence, as they began to turn on their heel.
However, Papyrus struck the ground with a fist, causing some more of his dust to flutter into the air. "No!" he shouted out. The kid snapped their head back toward him, stopping in their place. "I-I may be turning into dust, but... I can feel something in my SOUL––some burning feeling. Whatever it is, it won't let me die." Papyrus paused for a moment, narrowing his eye sockets at the human. "This ain't just about monsters anymore, is it? If you continue on, you-you won't stop, will you? You'll just kill everyone!" His voice became louder, edging into a growl. "Even the innocent ones like Un––!" he choked on the name, taking in a gulp of air as he squeezed his eye sockets close again. "Like Undyne...
"Everyone's hopes... everyone's dreams... gone just like that. But I won't let that happen!" Papyrus opened his eye sockets as he raised himself as best as he could on his shaking arms, his lower end now mostly dust. "Right now, I can feel it: everyone's SOULs beating as one, everyone having the exact, same goal: to defeat you."
Papyrus stared straight into the human's sanguine eyes, his voice portraying his undeniable determination. "Whatever you are..." He clawed at the ground, pulling himself closer to the human. The human took a step back as a response, eyes slightly widening. "For the sake of the whole world..." The dust surrounding him wavered, pulling itself together to create what looked like his old legs. "I, the Great Papyrus..." His left, blind eye socket suddenly flared up, orange, magical flames sprouting from its depths. "Will strike you down!"
A bright, white flash suddenly encompassed Papyrus, the human having to cover their eyes due to the sheer intensity of it. When the light finally faded away, they looked back at him, a faint gasp escaping their lips. Papyrus, complete and whole, now stood straight and proud as a wide smirk filled his face. Instead of his dented armor, he now wore gleaming, undamaged armor with sharpened bones adorning the shoulder pads and an orange heart painted on the breastplate. With a bark of laughter, he summoned a magical, blue bone.
"You're gonna have to try harder than that!"
(*The true hero appears.)
The trumpets and drums reached their climax once again as the human dodged the rising bones, getting clipped once in the shoulder. The bones eventually receded, and the human struck, landing a critical hit right on Papyrus' ribcage. He staggered but quickly regained his footing.
With a guffaw of laughter, he summoned a Gaster Blaster and shot at the human. "Is that the best you––?!"
Clang!
The flame in Papyrus' left eye socket was extinguished as the human landed another kick, forcing him to the ground. His weapon flew out of his hand and dematerialized as soon as his spine connected with hard wood, making him sharply inhale out of pain. As he lay, he saw his HP bar deplete completely above him. The human had done it. The human had bested him.
He could literally feel his legs turning into viscous, effervescent goop and bits of his skull dripping off of him––and yet a cackle escaped him as he sat up as best as he could.
"You think you won?" he hollered, his grin from before still in place. "Before I came here, I told Sans to watch over me. He's seen everything." Another cackle escaped him, edging into hysteria. "By now he's evacuated everyone and told Asgore to absorb the six human SOULs. Asgore is literally a god now!" A bubble of laughter escaped him. His mouth was beginning to stick together, obstructing his speech. "Don fu ged id?" He moved somewhat toward the human, trembling incessantly as he slumped a smidge closer to them. "F's ober!" he exclaimed. His laughter by then had diminished into an energetic bubbling, his face warped to the point of him no longer being recognizable. "Blober!"
With one final swing of the leg, the human kicked the gloppy remains of his skull off his melting body, obviously tired of his ramblings. His last gurgles reverberated against the walls until a splash could be heard, effectively ending his cachinnations. His "body" finally gave way, landing completely against the bridge. The goo dripped out of the gaps of the planks until nothing but black residue remained.
(*YOU WON! Earned 1500 EXP and 0 Gold.)
The human's SOUL returned into their chest, and they regained their color. Then, as if nothing had happened whatsoever, they walked back down the bridge toward their SAVE point. They stopped at the end of the bridge and looked back at it. The howling wind before had stopped since then, and, now, only silence filled the void.
It filled them with determination.