Black Hearted Determination

by I Ate Appiness

Rating: teen
Tags: Adventure, Undertale, Pinkie Pie, Crossover, Gore, Violence, Princess Celestia, My Little Pony: Friendship is Magic, Dark, Death, Alternate Universe, Human, Main 6, Tirek, Princess Luna
Length: 2614

The underground is in ruins, monsters evacuating left and right and dust filling the paths leading to the King's domain. Only one monster is left to stop the rampaging human before it's too late. The judge, Jury, and Executioner: The Smiley Trashbag.

Countless deaths and many save later; Frisk was finally getting the edge, forcing Sans onto the backfoot. But right when the human was about to deliver the final blow, a savior dropped from the skies. Snatching him away from such fate and bringing him across spacetime into a new world. A new life. However, he is not alone, as the threat of humans arrives in this foreign land as well. Their blade thirsting for blood.

With nothing to his name and bringing along the scarf to remember his brother by, how will Sans go about in this alien world?

An Undertale/Mlp crossover takes place in the Genocide route of the game. Spoilers for The Undertale Game.

Published 2024-10-09 03:30:00 UTC
Last updated 2024-10-09 03:30:00 UTC

"Heya, Pinkie...

Deep beneath the surface, the earth groaned with an unseen burden. Miles down, carved from the living rock, lay the Judgement Hall of The Underground. Once, perhaps, a place of order and bureaucracy, it was now a desolate wasteland. Scattered remains of bones lay strewn across the checkered black and white floor, littering the base of towering, crumbling pillars. The very air hung heavy with the weight of forgotten souls and untold injustices.

In this desolate chamber, two figures stood tall. A lone skeleton stood slumped against a pillar. He wore a tattered blue hoodie, far too large for his frame, black basketball shorts that hung loosely from his hip sockets, and a pair of incongruously pink, fuzzy slippers. A red scarf, its fabric faded and threadbare, was wrapped haphazardly around his neck.

On the other side of the corridor stood a human child with a devilish grin. No older than thirteen, perhaps.

With a snarl, the human lunged, aiming the knife brandished in their hands straight for the skeleton's chest. The blade glinted faintly in the dim golden light. But the skeleton, despite his apparent slumber, seemed to possess an unnatural awareness of his surroundings. It doesn’t help that the child was yelling their heart out.

With surprising agility for a creature devoid of muscle, he twisted to the side mere inches before the blade connected. The knife sliced harmlessly through the empty air, its intended target replaced by a crumbling pillar.

"Heh!" The skeleton laughed, sliding out of the way with a shrug. "Did' ja really think you would be able-"

A dusty knife quickly cuts him off as it nears the skeleton's chest, the cold metal grazing his white undershirt. Sans stared into the kid’s heartless eyes. He knew it was the end.

However, it seemed the universe had other plans for him as a portal boomed up above them, dropping a blue pegasus with a rainbow mane and tail onto the child. The mare crashed into the kid in a dynamic tumble.

"Oof!" Frisk cried out, only for their voice to be drowned out by the clatter of their knife as it harmlessly fell onto the tile floor. They writhed in pain, growling like an animal beneath the actual animal lying atop them.

The pony seemed to be out of it as well, bringing a hoof to rub circles on her head. The disorientation was overwhelming.

"Wha...?" The mare mumbled, confusion evident in her voice. She brought their other hoof to examine it, and a gasp escaped her lips. She was even surprised by her current condition. "Oh, c'mon!" She bemoaned, her long tail swished nervously behind her. "Again!?"

"Get. Off." Frisk grunts, pushing off the ground with both arms in order to get the mare sitting atop them to fall off.

Sans grinned, raising a hand and warping his magic around their soul. A strange sound pierced the air – a high-pitched "Ping!"

A blue light emanating from somewhere unseen illuminated the area beneath them. Swirling with energy, their soul materialised inches from their chest before abruptly falling downwards. The human, caught in the invisible current, was dragged along with the spectral heart. Their grunt was cut short as they slammed into the hard floor with a crack, their control over their body being voided and nulled as Sans chuckled.

"I don't think so, buddy." He mocked, his left eye glowing a similar blue. Thanks for the assist, Blue-y." Turning to the blue pegasus monster, he shot her a thumbs up, a smile plastered on his skull. "I got it from here."

"My name's not Blue-y." she deadpans, to which the skeleton shrugs—a good nickname if you ask him.

From the chaos above, a human wearing a white jacket and navy blue, borderline black pants while carrying a sword tumbled down from the still-swirling portal. He landed with a bone-jarring thud beside a scowling monster. Dust motes swirled around them as the other human scrambled to his feet, a monstrous white specter looming protectively behind him. Before he could react, the monster with the burnt face lashed out, a vicious kick aimed at the boy’s face.

The force of the blow had sent the boy reeling back, winds whipping past him with a harsh whistle.

Sans watched on with a raised, non-existent brow. They were really going at it, the boy fighting back at the monster with such ferocity that it reminded him of the trapped kid in front of him. It was pretty bloody, too. Limbs exploding and skin tearing open.

“Welp, they look like they got their own business to attend to. Speaking of business,” Sans approached the human trapped beneath Blue-y, eye still glowing. “Ours hasn’t concluded.”

Right as Sans was about to deal the finishing blow, a sudden rolling force from behind smashed into him. Glancing back, it was the human, with the two other monsters in a heap of rolling bodies tackling him. From the direction they came from, even the pegasus and Frisk weren’t clear from the route of bodies as Sans slammed into Blue-y, while Frisk was dragged along for the ride.

The impact sent all six of them tumbling in a heap of limbs and confusion. In the pandemonium, another swirling portal manifested in front of them, a desperate escape route. With a final, triumphant roar. The monster the boy was fighting steered the rolling ball of bodies into the portal, dragging the chaotic mess along with them. The portal snapped shut behind them, leaving only the echoing silence and the lingering stench of burnt flesh and despair.

“W-woah!” Sans yelped, holding on for his life to the giant ghostly apparition next to the human boy.

The group shook left, then right, then up and down in the swirling mess of the wormhole. His head was spinning, and his sense of vertigo was all over the place. But he wasn’t that confused to be unable to catch a glimpse of Frisk lunging in from the side, swinging the knife at him.

Sans’ grip was waning, his fingers losing strength from his earlier tango with the human in The Judgement Hall. A sweat breaks from his cranium. If he’s going out, minus well drag the kid down to hell with him.

Sans released his hold and reeled back, narrowly dodging the vertical slice of the kid’s blade as it cruised through the air with a whistle. Pulling an arm back, Sans throws his palm into Frisk’s chest. A bone manifests along its path, shooting through the air at tremendous speeds and punching it into their gut.

“Ourgh-!” Frisk puked, saliva spewing from their lips.

The force of the attack was more than enough to launch them into the side of the dizzying wormhole. Frisk tried to reach out to them, but no one reached back. A scream tore from their throat as they phased out from the light and into nothingness, not a single remaining artefact of them surfacing. Sans sighed. The Underground was safe, at least. He feels… content with this, body going slack as external forces drag him into the void.

Sans turned his head for a fleeting moment, his empty eye sockets relaxed as they landed on the mare in a gesture that could be interpreted as either "I'm glad that's over." or a "Take care.".

"Skeleton dude!!" Blue-y shrieked, a note of genuine sadness lacing her voice. She reached out a hoof, desperate to grab onto something, anything, in the swirling chaos.

Sans smiled. He never thought that monster horse would be this attached this quickly. Welp, the worst is over now. He closed his eyes as the light washed over him. Enveloping him entirely, drowning out everything except for the heartbeat of his soul.

“Papyrus…” Sans clenching onto the scarf wrapped around his neck, dreaming of a future far from his reach. He chuckles, perhaps in another life.

Sans sank through the darkness in silence, a lead weight plummeting through the ocean depths. A distant, muffled voice tried to pierce the silence, but he couldn't discern the words. After what felt like an eternity of falling, Sans landed on a soft surface. Unmoving. Another sound came from all around. It was the muffled voice again, this time slightly louder than before.

It seemed to want to reach him. Moreover, it sounded… worried. Worried for him. It seemed to want to reach him. Sans slowly blinked his eyes open, adjusting to the sudden light. The first thing to greet his vision was a vibrant blue sky dotted with fluffy white clouds. The second was a bright pink pony, her curly mane swaying in the gentle breeze, looking down at him with worry.

“Are you okay?” The mare asked, her curly mane bouncing in the gentle breeze. “You’re sweating a lot there. You don’t look so good.”

Sans shifted, sitting upright as the mare stepped back, gaining a better view of his surroundings. He found himself resting in a grassy field, the warm sun bathing him in its golden light. Birdsong filled the air, and colorful flowers bloomed around him. He didn’t think the afterlife would be this peaceful.

“Where am I?” Sans finally spoke. Rubbing the back of his skull while feeling his mouth dried out.

“You’re a few stops away from Ponyville,” The mare said, scooting over to reveal to Sans a set of train tracks passing through a train station just a short walk from where they were. “I was actually heading back home, that’s when I saw you through my window seat lying in the grass and thought you looked… sad.” The mare broke off eye contact, fidgeting with her hooves and glancing to the side before turning back to face the skeleton. “Is everything okay?”

Sans looked around once more, taking in the fresh air. It was far purer than anything he had ever experienced in the Underground. The vibrant colours of the world above were a stark contrast to the dull, grey tones he was accustomed to. "Papyrus would love it," Sans thought. Then, a sudden realization struck him. "Wait... Papyrus?"

A hand shot up to his neck, and a wave of relief washed over him as he felt the familiar fabric of his scarf. It was still there. Good.

"Is that scarf important to you?" the mare asked, tilting her head to the side.

"Uh, you could say that..." Sans looked down for a moment before turning back to face her. "I'm holding it for my brother. He's quite the busy skeleton."

"Oh, where is he? Maybe I can help you look for him," Pinkie offered, turning around and squinting her eyes in search of the mysterious skeleton brother.

"I doubt he's here," Sans replied, standing up in his fuzzy pink slippers. Now, at a respectable four feet tall, he matched the mare at eye level. "Like I said, he's a busy skeleton. He'll find me when he wants to. Anywho," Sans reached out a hand, “I’m Sans, Sans the skeleton.”

The pink earth pony looked down at his hand. She smiled, reaching out with more energy than Sans could muster. “I’m Pinkie Pie-!”
Her introduction was abruptly cut off by the wettest fart coming from the palm of Sans’s hand. “The old whoopee cushion in the hand trick,” The skeleton’s smile grew an inch wider, his hand vibrating from each gastric release that came from the device in it. “It never gets old.”
Pinkie burst out in laughter, flopping back onto the grass and clutching her belly, her hindlegs kicking the air. “You got me good!”

Woah, Sans knew he was a comedian, but it's been a long since he got this reaction from somebody… or Pony in this case. Other than that old lady behind those doors on the far edges of Snowdin.

“What can I say?” Sans shrugged, “It’s a profession.”

Pinkie snickered, getting up. “You’re funny!”

“I get that a lot.”
“Do you have anywhere to go?” Pinkie asked, looking around. “I don’t see a house anywhere.”

“Nah,” Sans shook his head. “You could say I’m travelling around.”
“Then, do you want to visit Ponyville?” Pinkie suggested, bouncing on the spot. “It’s a wonderful place with lots of fun people. Just like you!”

“Sure,” Sans replied, “a place full of me is sure to be a good time.”

Sans followed Pinkie down the gentle slope, his slippers crunching on the blades of grass as he took in the scenery. It’s beautiful, something he couldn’t put into words how elated he is. He’s sure Papyrus would enjoy this, too. Reaching for the scarf around his neck, he folds it neatly before tucking it away in his jacket.

Continuing their trek, they reached the train station with little to no hiccups. Sans stepped up the flight of stairs onto the elevated boarding floor as Pinkie hopped behind him.

“You sure are happy,” Sans commented, turning to face Pinkie. “Something good happened?”

“I went off to visit my family!” Pinkie stopped hopping. “It was nice to see them again after some time! What about you?”

“I get whatcha talking about,” Sans chuckled, “There’s no better company than my bro. As ecstatic and energetic as he is, he always manages to put a smile on my face.”

“Oh wait!” Pinkie stopped. “You’ll need to get a ticket to board the train.”

“Uh… hehe,” Sans begins to sweat, his thick hoodie doing him no favour in the situation. “I’m a little short on cash.”

“Give me a sec!” Pinkie hopped over to the counter located near the station cabin. After exchanging a few words, she hopped back with a ticket in her mouth. “Here ya go!”
Sans took the ticket from her, examining the little red slip of paper. “Thanks, Pinkie. I’ll pay you back once I get the gold.”
“Bits, you mean?”

“Uh, yeah. That.”

Right on queue, a horn in the distance signals the arrival of the train. The massive hunk of steel rumbled, groaning to a stop before the station floor and opening its doors. A few ponies alighted the train with their luggage or bags in tow, giving Sans and Pinkie the right of way aboard the vessel.

Pinkie plopped down next to a window seat as Sans took the one opposite to her. The train whistled, and the behemoth began to move. Sans leaned against the window, watching the plains shift before him as they passed into more plains.

The skeleton took a deep breath in and exhaled. There’s no way this is the surface; humans wouldn’t allow talking ponies to exist in the same world as them. Was this his second chance at life? What a way to go about it.

“Is something wrong?” Pinkie asked.

“Nah,” Sans replied, staring off into the window. “Just sightseeing.”

“Well, there’s more to see in Ponyville! You’ll like my friends, I’m sure of it! There’s Rainbow Dash. She’s the fastest pegasus I know! She created something called a Sonic Rainboom by going really, really fast! And, and, there’s Rarity! She’s a fashion designer, and…”

Sans smiled, turning to lean on his wrist instead and face Pinkie. She reminded him so much of him; she was so cheerful, optimistic, and energetic. Sans closed his eyes, taking in all the things she was spewing at him. Not once getting drowned in the torrent of words. It turns out that listening to Papyrus ramble on proved to be a good listening skill.

Author's Note

I contemplated if I should've stayed true to the game by having all of Sans's dialogue be written in small caps. But for my sanity, I chose not to.
Also, thank you for reading!