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Published:
2026-07-31
Updated:
2026-07-31
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1,653
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1/30
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How To Screw Up Space and Time (and Seriously Regret It)

Summary:

After Bill Cipher gets defeated by the Mystery Twins in Stan's mindscape, he starts to realize that these kids are way tougher than he thought. So, what does the Triangle Head do?

He decides to teleport from Gravity Falls to a far away universe set in Taisho Era Japan so he can team up with the Demom King, Muzan Kibutsuji. Bill thought it would be easy: traverse timelines, strike up a deal, kill their enemies, return to a Pines-less Gravity Falls, and prosper.

But unfortunately for him, the Mystery Twins and their Grunkle Stan ruined Bill's teleportation plans. By short-circuting the Interdimensional Portal, instead of teleporting across realms, Gravity Falls and Demon Slayer merged instead.

Will Dipper, Mabel, and their new friends be able to team up with their new Demon Slayer buddies to stop Bill Cipher and Muzan from collapsing their entire sense of reality in time?

What?

Don't look at me―I'm just the narrator!

Notes:

I might be addicted to crossovers now.

Oh well. Hope you guys enjoy!!

In this fic, Dipper, Mabel, and Pacifica are 13 while Robbie and Wendy are all 16, and Tanjiro is 15 (he's almlst 16--this fic takes place in June and Tanjiro's bday is in mid-july). Senjuro is 13, and Ume was 13 when she became a demon, so Daki is ALSO 13. Lets also just pretend that Robbie and Tambry broke up (i have nothing against that ship its js for the sake of this fic!)

-Blu3_Halo

Chapter 1: If Anyone Asks, It's WADDLES' Fault

Summary:

A spill causes Gravity Falls citizens to be flung all across the Demon Slayer universe.

Chapter POV:
○ Bill Cipher POV

Chapter Text

Kaboom!

 

The impact from the explosion echoed throughout Gravity Falls, shaking the entire city and everything in it. There were screams for panic and falling buildings. The air crackled with an overwhelmingly dangerous energy that threatened to obliterate everything in its path. No one knew what was happening exactly, but the moving clouds overhead filled everyone with a crushing feeling of doom.

 

Before anyone could register what was going on, a swirling, blue light appeared in the middle of the circling clouds. It was like some sort of whirlpool or black hole. Suddenly, all the screaming and chaos stopped. The portal sucked everyone in Gravity Falls up like some overpowered vacuum cleaner.

 

All was silent. With a satisfied slurp, the portal closed, and the light disappeared.

 

All was quiet in the ruins of the city. Little did anybody know, it was about to get a whole lot louder somewhere else.

 

 

...~.~...

 

 

That stupid, rotten pig…

 

That was all Bill was thinking as he plummeted down from the blue swirl of light consuming the sky above. His eye was narrowed in frustration and hunger. 

 

Hunger for what exactly, you ask?

 

Well, for food, duh. He had just attempted (and semi-completed) to kickstart a really taxing mission, and he was seriously craving donuts right then. Especially the chocolate ones that--

 

But never mind that right now! It wasn't completely hunger for donuts. More importantly, it was hunger for vengeance.

 

And Bill Cipher was going to get it no matter what.

 

Everything had been going perfectly: Bill had been able to possess Ford Pines again, and then he was able to find the Interdimensional Portal in the bottom of his lab. All he had to do was press a few buttons, and then he would be able to traverse dimensions and recruit the feared Demon King, Muzan Kibutsuji.

 

Bill had been cooking up this plan for a while now. After weeks of frustration of looking into threats in Gravity Falls he could release on the Pines family, the pyramid had stumbled upon an old scroll of cautionary tales. Bill had spent hours reading about all kinds of terrifying monsters that had supposedly wiped out hundreds of thousands of souls.

 

But one tale stood out from all of the rest.

 

An old Japanese fable originating from late in the Heian Era about bloodthirsty creatures that fed on the blood of humans. 

 

Demons.

 

The more Bill had read about them, the more he knew he needed these creatures on his side. They had superhuman strength and speed, could regenerate their limbs in the blink of an eye, and could even develop other worldly powers called 'Blood Demon Arts'. Their only weakness was the sunlight, decapitation by a special kind of sword, and the feebler demons could be killed by Wisteria poison.

 

In short, these monsters were supernatural beings with only a handful of weaknesses. 

 

Bill had immediately formed a plan:

  1. Teleport to the demons' universe (he knew exactly how to do that)
  2. Find the demons' hideout (some place called the 'Infinity Castle'?)
  3. Make a deal with Muzan Kibutsuji (he might need to write a villain monologue or something convincing for that part)
  4. Complete his end of the deal (whatever that might be)
  5. Come back to Gravity Falls (might be a little harder than the first step)
  6. Take revenge on the Pines family (that will be fun)
  7. Wreak havoc for eternity (without having to worry about a set of meddling twins getting in his way)

 

The pyramid desperately needed this plan to work, so before it could even begin, he had to set it in motion. Bill had accounted for every variable that could compromise his plan. Even if the Mystery Twins tried to stop him, he would be ready. 

 

Except, in all of the scenarios the pyramid had gone through and worked out in his favor, he hadn't expected a pig to be added into the equation.

 

Yes. The Almighty, All-Powerful, All-Seeing Bill Cipher had been defeated by a pig. Well, technically a soda and a pig--eh, it's complicated.

 

Anyways, here's what happened:

 

Bill flung open the laboratory door, catching sight of the machine in front of him. Bill stretched Ford's face into a triumphant smile and let out a cackle. Because Bill was currently possessing Ford, both his and the pyramid's voices twisted and contorted into one malevolent frequency.

 

Bill manipulated Ford to take a confident step forward. But the pyramid had still been adjusting to possessing a body with a different anatomy like he always had to, so Ford's leg missed the first step of the staircase completely. 

 

Ford's body tumbled down like a ragdoll, and Bill felt every collision with the laboratory stairs as clearly as crystal. 

 

Once Ford's body hit the floor at the bottom of the steps, he grumbled in pain. Bill forced the body to stand up.

 

"Human eyesight really sucks…" Bill muttered in frustration. It was always a real chore for the pyramid to adjust to having two eyes to see out of instead of one whenever he took control of a human body.

 

After Ford's body stood up, Bill's eyes widened as he stared at the machine in front of him.

 

This was it.

 

The Interdimensional Portal.

 

Bill let out another villainous laugh as he approached the control panel. But before he could even lay Ford's hands on it, he heard footsteps racing towards the lab.

 

"Stop right there, you--uh…you pyramid head dummy!" 

 

Bill clenched Ford's fists in frustration. He recognized that voice. The pyramid-shaped possessor whipped around to see Stan, Soos, and the Mystery Twins standing in the lab doorway. Dipper was standing like some Disney hero, pointing right at Bill accusingly with narrowed eyes. A bead of sweat trickled down his forehead, signaling that he was nervous.

 

Soos cast a side eye to his friend as took a big gulp from his soda can.

 

"Dude. Weak." He deadpanned.

 

Dipper stuttered, his face flushing in embarrassment as he turned to face Soos.

 

"J-Just--put the soda down, Soos!" Dipper eventually squawked out. Soos just shrugged and placed the soda can on one of the tables in the lab after he made his way down the stairs. 

 

“You guys should turn right back around before you get hurt.” Bill warned maliciously. Then he smirked. “Or you all could stay. It’ll be fun to put you brats in your place.”

 

“That’s not gonna happen.” Mabel announced as she stepped forward, hugging her pig, Waddles, against her chest. “Because we’re gonna stop you first.”

 

Waddles snorted, agreeing with Mabel’s statement. Stan grumbled, pinching the bridge of his nose as he walked up to stand next to her.

 

“Mabel, please, put the pig down.” Stan sighed. Mabel nodded, narrowing her eyes as she gently set Waddles down next to Soos on a nearby table, never breaking eye contact with Bill.

 

“Soos, watch Waddles for me. I’ve got some butt to kick.” She said, cracking her knuckles.

 

Bill only laughed maniacally, his voice merging with Ford’s. A sly smile spread slowly against Ford’s face, his eyes narrowing.

 

“You think you can stop me? You’re too late! Even if you had come in time to beat me here, you wouldn’t stand a chance anyways! I’m stronger, smarter, more powerful than you could ever be—”

 

Bill cut off his villain monologue after he spotted something out of the corner of his eye.

 

It was Waddles. Who was trotting towards the half-full soda can. The soda can that was on the edge of the metal table.

 

The metal table that was right next to the portal control panel.

 

Bill’s breath hitched, Ford’s eyes widening as he turned to face the pig. Waddles was now right next to the can, sitting idly like there was nothing remotely important or life-threatening happening right then.

 

“Uhh…good piggy…nice piggy…” Bill started, gently stepping towards where Waddles sat on the table. The possessor stretched out his hand to grab the soda can and take it out of the pig’s reach. Waddles finally noticed the can next to him and stared at it for a long moment.

 

The Mystery Twins, Stan, and Soos caught on to what was happening. Mabel’s expression grew concerned as she reached for her pet pig.

 

“Waddles, no. C’mere. Come to Mabel.”

 

Waddles looked back at her. Then at Dipper, Soos, and Stan. Then at Bill, and finally, back at the soda can.

 

Right before Ford’s hand could snatch the can away, Waddles lifted his arm, and knocked the soda over.

 

Right onto the control panel.

 

“No!” Bill yelled in panic, running to the panel. The Pines family took a step back as the Interdimensional Portal started to crackle with electricity. The machine started to spark all over, and the motherboard started to smoke.

 

“Watch out—” Soos yelled, jumping back. “IT’S GONNA BLOWWW!!!

 

And then, the portal exploded, throwing everyone back against the wall of the laboratory. Stan groaned as he clutched his stomach, but then he looked up to see a flash of blue, swirling light right where the machine used to be.

 

And then, before the Pines family and Bill could even register what had just happened, they all got sucked into the circling light.

 

Outside, screaming and crashing could be heard. But just as quickly as it came, silence fell over the destroyed city of Gravity Falls.

 

The magnitude of the portal had forced Bill’s possession of Ford to cease, spitting the pyramid out in the middle of the sky.

 

As he fell to the ground hat-first, he clenched his fists.

 

That stupid, rotten pig…

 

The point of Bill’s pyramid head wedged into the dirt below, but he didn’t even notice. He was overwhelmed with embarrassment, frustration, and an uncontrollable tidal wave of fury.

 

No. This was not the last the Pines family would see of Bill Cipher. And when he got Muzan on his side, Bill would make that pig pay.