Bill Cipher is an otherworldly demon trapped in a nowhere dimension. A colorful emptiness. The ethereal barrier between reality and other dimensions are thin. Every time he makes a deal Cipher is able to withstand staying in the physical world for a longer time. If he wants to take over Gravity Falls –and eventually the entire world, he's going to need an army. An army of bloodthirsty, mischievous, apocalyptic, and sinister beings.
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Recruiting from other dimensions is integral. With his cane Bill tears a portal through the endless colors and creates a door to multiple universes. Should he summon monsters? Cipher laughed. No. It'd be better if they were villains like him.
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Snapping his fingers invitations are sent. Elegant ebony envelopes sealed with golden wax. What kind of villains would attend his party? Giddy about earth's eventual destruction he floats around his territory, creating the best party in demonic existence.
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A swanky nightclub appears. Floor and walls are black marble outlined in gold. Neon lights change color, shaped like mythological monsters. Deafening music plays from pyramid-shaped speakers. Drinks and snacks are important too. Bill had prepared for every type of guest. And hell, if they wanted to burn it all down afterword he'd be flattered. All great shows end with explosions!
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Five people arrive and are chucked out of the portal onto icy ground. Each guest looked as bizarre as Bill Cipher. The demon laughed and they all stared at him. "Welcome! Slashers, villains, antiheroes. I'm Bill Cipher and this is my dimension. I've invited you here because you have talent. Tell me who you are!"
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"Is this the afterlife? It's a lot better than I thought it'd be!" A teenager with short hair and a wild smile exclaimed. He looked at the club. "Our host knows how to party!"
His friend crossed his arms and glared at Cipher. "Are you a demon?"
Bill bowed, removing his hat. "Yes, the best there is!"
"Is this just another part of the Other World?" A bored-looking man sighed. Yet underneath his dark eyes Cipher sees a glimmer of curiosity. "None of you look like shadows though. So other dimensions besides the Midnight Channel exist huh?"
"Ryuk," a brown-haired man dressed in a red tie and beige suit asked the air. "Is this the Shinigami realm? It can't be. There are too many annoying colors."
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Bill snapped his fingers and a long table that could fit a whole kingdom floated down. Seats followed. "Drinks? Snacks? We have a lot to talk about."
What does this strange demon have to offer? The dapper triangle waved an arm, signaling the others to introduce themselves.
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The tired cop is the last to talk. "I'm Adachi from Inaba. I'm a detective." He nonchalantly talked about shadows and the Midnight Channel. He's on the run from some meddling mystery-solving kids.
Light narrowed his eyes at Adachi. "You're a detective and a murderer?"
He smiled sarcastically at Yagami and responded in the same mocking tone, "And you're a police chief's son and you kill people."
"It's for justice! Only criminals and those who refuse to accept the new perfect world I'm creating are killed."
Adachi chuckled. "Good and bad? What selfless morals you have, Yagami! You're like those kids blindly running around the Midnight Channel acting as if they know what true justice is. Your ideals are funny. Face it, kid. You and me? We're the same. Bored."
Ryuk cackled. "He's got you there, Light."
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"It's cool how a notebook can do all that." Billy said. "You can make people die however you want them to."
"It'd be less fun though." Stu said, drawing his fingers over the knife thoughtfully. "All you're doing is writing names down."
Yagami cleared his throat and looked at the demon. "Why'd you invite us here, Cipher?"
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The demon held a martini glass in his hand, heavy globs of blood swirling in the glass. "With all of us together, as cliché as it sounds, we can take over four dimensions. Completely destroy them. And after, recreate it however we want. Why do heroes win? I say we're heroes, not them."
The guests murmured in agreement.
"I say we show them something to be scared of! A true, inescapable, heart-stopping horror. We'll be the ones to change the worlds!"
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"What do we have to do?"
Stu elbowed Yagami lightheartedly who just returned a resentful glare. "Yeah. Light over here can just write all our enemies' names in his notebook."
"We don't want to give away all our moves. Light will do what he's doing in his world. As Kira, he'll kill those as a last resort –to scare any brave heroes that try to defeat us. We're going to start in Gravity Falls."
"Yeah?" Billy crossed his arms. "Your world. How do we know you aren't going to trick us?"
Adachi nodded. "It won't be any fun if we don't get our share of this new world."
Cipher laughed. "I wouldn't trick anyone who's as evil as me. I need all of you if this is going to work. Here's our plan."
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Waving his cane, visions appear. "Stanley Pines is the owner of the Mystery Shack. Behind a vending machine are stairs that lead to a portal and a lab. You can turn on the machine and get the interdimensional rift and use it as a door to your world, Adachi. The shadowy substance that the machine emits will give me extra powers. Stanley has been trying to rescue his brother from another world but we won't let that happen. Sixer would be a dangerous enemy to us. Stu and Billy will enter Adachi's dimension and stop the investigation team. If the portal is open they'll try and find a way through to Gravity Falls. Then we…"
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Adachi parked outside the tacky tourist trap. He smiled, studying the Mystery Shack for a moment. Gravity Falls is a lot like Inaba. He opened the car door, got out, and strolled up the steps. Entering the empty shop, Adachi met Stanley who gave his best showman persona. And he's like Dojima. Adachi knew that look. Stanley guessed he's an agent.
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"Looking a little fancy for stopping at an out-of-the-way tourist shop, yeah?"
Adachi gave a disarming smile. "Oh I just got lost. Looking for directions."
Stan grumbled. The faster this guy gets out of here the better. "You're pretty far from anything. Maybe you should stop in town for the day." The conman walked over by some old TVs and flipped through dated maps.
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Quickly the cop looked at the TVs and found one that he could push Stan through. Before teleporting to this world Bill had tested to see if their powers still worked. His did and Adachi was excited to use it. He leaned against a shelf as Stan handed him a dusty folded map. Adachi grabbed hold and smiled. "Thanks. Stan Pines right? This shop isn't so bad."
The man humorlessly chuckled. He's about to brusquely reply when Adachi lunges, pushing Stan back.
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Pines expected painful contact with his jaw to the TV screen and sharp glass to cut his face –instead he leans through the screen and it felt like he was crashing through a pool into depthless darkness. Losing his footing and surprised, Stan plunges into the Other World, screaming. Adachi ran to the vending machine, copying the moves Dojima used when breaking through locked doors. The metal screeched in protest but moved.
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"Mr. Pines? I heard you scream. Are you okay? I –oh my God! Who're you?!" A red-haired lumberjack girl and two twins stared at Adachi, shocked. He didn't answer and ran down the steps, almost tripping. Panicked footsteps followed. Adachi's heart is pounding and he wheezed, laughing.
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The giant portal loomed over the capacious lab. It's cluttered with dusty old equipment and for a second he wondered if any of it even worked. Adachi jumped to some controls, remembering what Cipher showed in the visions, and started the machine. Energy hummed and popped deafeningly. Stunned my immense light Wendy, Dipper, and Mabel are thrown up into the air, weightless. Adachi leapt and grabbed the metal gate. He dipped his hand into the white-blue light, changing it to swirling black-red stripes. Dark bubbling energy leaked out from the energy and he collected rift fragments into a glass flask.
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"Who are you?!" Wendy shouted.
"What did you do with Grunkle Stan?!" Mabel glared at the grinning cop. Shadows escaped from the portal. They gurgled menacingly as they crawled towards the heroes. "W-What are those things?!" Dipper hasn't seen anything like this in his adventures or in his trustworthy journal.
Adachi held a floating card in his hand. He felt the atmosphere become dense and almost stifling just like the Other World. Mist saturated the air too. "Nice to meet you," he grinned cruelly. "I'm Adachi and I work with Bill Cipher. Good luck surviving in the Other World! Persona!"
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Magatsu-Izanagi appeared behind the heroes. Wendy hurls her axe at it only for the weapon to go through the demon harmlessly and hit the wall. Mabel fired her grappling hook but Magatsu-Izanagi's sword cut the wire and he flung them into the portal. Adachi laughed. "I wonder how much chaos will befall these heroes. This'll be fun." Looking up he shouted, "Cipher! Send me back to your world. It's up to Ghostface now!"
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In Inaba, Billy and Stu had been tracking their enemies for weeks. They went unnoticed even in such a small town because Dojima and Yu were trying to find Adachi who had escaped. Sinister mist from the Other World was slowly waning peoples' minds and bodies. Both dressed as Ghostface, Billy wiped blood from the knife. Rise didn't expect them. She had sprinted down streets being chased, hoping to run into Yu and the others but met a knife to her heart instead. Billy departed to Junes while Stu waited for the Investigation Team.
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They had found pools of blood leading up to the tree –and Rise. Ghostface tauntingly jumped off of the table and waved his knife. Yu, Kanji, Yukiko, Chie, and Yosuke ran after him. They lost him in the aisles of Junes but Yu saw a flash of dark cloth by a shelf.
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The TV they use to enter the Other World flickered with static. Bill Cipher's wheel glowed on the glass surface and Ghostface walks through the portal. The Investigation Team jump in and fall down into Bill's nightclub. Shocked, they summon their Personas but eyeball monsters quickly turn them into stone.
Light Yagami sat at a desk and flipped through the death note. "Should I write their names down?"
Bill twirled his cane. "Not yet. They've turned to stone. We've got other things to do."
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Demons and shadows sieged Gravity Falls. Fires highlighted the darkening landscape as eyeball monsters turned citizens into stone. Cipher had summoned a car to oversee the destruction. Adachi, Billy, Stu, and Light got in and the car flew up into the sky.
The heroes might return to fight once more, as heroes often do, being stalwart adversaries. But Bill and the Bad Guys Club are ready to fight if they somehow escape their eternal prisons.
Gravity Falls is no longer strangled by the laws of reality or society's superficial ideals. Now the villains are the gods of the new world.