Based on an idea by cosmicpines on tumblr. All credit for the idea goes to them!
Those idiot twins thought they had him. The girl, especially.
He hadn't been lying when he'd said it was hilarious how stupid they were. Did Shooting Star really think that she could do anything to stop him, a being of pure energy? Did she think that exhausting Pine Tree's pathetic body would have any real effect on him?
She was seeing what she wanted to see, blinded by her pathetic human need to think that she could change the world, could defeat her enemies.
Oh, but Bill was excellent at pretending, and more than happy to play along.
It was easy enough to mimic Pine Tree. No one but Shooting Star was even looking too closely at him, and with a few well-timed words, she believed beyond a shadow of a doubt that he was her brother, all effects of the possession gone.
And before the real Dipper could come forward and say otherwise, she was setting the puppets on fire.
It was hard not to laugh as Dipper's face crumbled, as the theater was sprinkled with flaming debris, almost a dress rehearsal for the end goal.
And that was it. A quick poke at his omniscience to tell Mabel he should go to someplace called a hospital, and Shooting Star believed that Bill was gone.
Dipper floated after them, shattered inside and silent. He didn't yell, didn't curse, didn't even cry. He was wonderfully placid, wonderfully broken.
Bill couldn't have asked for an easier setup.
In the town of Gravity Falls, things were weird. The residents had accepted it, or ignored it, or been driven insane enough to not recognize what was normal and what wasn't.
They didn't look twice at a kid with bright pink band-aids plastered across every limb, a result of Mabel's attempt at first aid. They didn't notice when he blinked one eye at a time, smiled too wide, laughed instead of cringed when he 'accidentally' hurt himself.
He was a bit more careful around Pine Tree's friends, especially that Wendy, who'd been giving Bill sideways glances pretty much the whole time he'd been in this meatsack. He made sure to use their names, and poked around in his knowledge of the universe, and in Pine Tree's brain, to find the right things to say—or right enough, at least.
As far as any of them knew, Dipper Pines was back.
And the real Dipper could do but nothing but watch as Bill set his plan in action.
Stanford Pines, paranoid as he had been, still hadn't found a decent password to protect the Portal. Bill cracked it easily, and the vending machine swung open.
Dipper floated nervously behind as Bill clambered down the stairs, just quiet enough not to wake anyone, just loud enough to have some fun.
"What is this place?" he heard Pine Tree ask, his voice soft. He didn't expect an answer, so Bill gave him one, just to mess with him.
"Let's just say it's a place your uncle and I used to work together," he cackled, ignoring the rest of Pine Tree's frantic questions about what Stan had been doing with Bill and why.
Downstairs, Bill checked out the dials and levers that made the Portal work. It would need fuel, but other than that was in good condition.
There wasn't much dust down here, either, and after a quick look around, Bill ran over to the first two Journals, plopping the third down next to them. Pine Tree's questioning got a bit more intense once he saw the Journals, but Bill ignored him.
The other twin, the one whose mind he'd been into more recently, had to be down here, getting the Portal ready, trying to bring his brother back.
How noble of him.
Surely he'd be getting the fuel sometime soon. No sense in Bill getting it himself, possibly blowing his cover and exhausting this pathetic meatsack in the process. He wanted to be there for the Portal's opening, in someone's flesh, so it could go just as planned.
Smirking, Bill snatched the third Journal back up and returned to the attic, smiling as Dipper fell silent and hopeless once again.
Waiting around in this meatsack was boring, but if he left it he wouldn't have another chance to get it back, so Bill put up with Pine Tree's moping and Shooting Star's energy (though it was refreshing to meet a mortal that could possibly keep up with him).
And he did get a few chances to do something fun, like watching the mansion of those snooty Northwests burn to ashes under the influence of a vengeful ghost. Some of the guests in the mansion at the time made it out alive, and it was hilarious hearing them moan and wail about their loved ones and friends turn into wood and burn to ash before their very eyes. Bill might have to try that when he got the chance . . .
But he'd also been sneaking down to the basement of the Shack, to the Portal, keeping an eye on the old man's progress getting it set back up. He was going pleasantly fast, now that he'd come across all the Journals, and soon the Portal's countdown clock was ticking.
The successful end of plans laid decades ago was coming, and Bill couldn't help but smile.
Pine Tree was curious, Bill had to admit. No matter how despondent and quiet he got, he still followed Bill around, still asked questions about what he was doing.
Bill ignored him. He answered him with lies. He answered with the truth. He did whatever would make Pine Tree the most paranoid, the most upset, the most terrified.
As far as Dipper knew, his uncle was working with Bill, had been for a while. Mabel didn't even notice he was gone, didn't really care about him. He was replaceable, he was forgettable, he was weak, he was useless.
Pine Tree spoke up less and less, floating after Bill limply, not even trying to communicate with his family and friends anymore, even when the opportunity arose.
Which was really quite perfect.
Grunkle Stan owned ten guns, scattered throughout the Shack, for various reasons. He didn't even remember where half of them were most of the time. He'd never miss one of them, if Bill were to take it . . .
So he did.
Finally, the day came, and it started with a bang.
Shooting Star had found illegal fireworks, and who was Bill to turn down a celebration of his success, especially a celebration so destructive? It was the success of his plans in miniature, a perfect start to the day.
The old man was in on it, too, laughing and having a fun time. As far as he knew, he was near success, about to see his brother again.
Admittedly, the police were not part of Bill's plan, but they didn't ultimately matter. Now that the fuel was in and the countdown had begun, Bill could start the Portal up himself, or even just wait for the countdown to reach zero. Better to be there, to make sure everything went smoothly, but in the end either way would work just fine.
Shooting Star was all for sneaking into the Shack, and it was all too easy to lead her to the basement, though they picked Question Mark up along the way. From there, it was smooth sailing.
"Whatever this is, Grunkle Stan must want it to happen," he said.
"Of course we should trust him," he said.
"If it was dangerous, he wouldn't be doing it near us; he loves us," he said.
Dipper was screaming in the corner about how he was lying, lying, no Mabel shut it down don't listen to him, but it worked all the same.
The three of them gathered around the controls to wait, Stan joining them as he burst through the door, panting. They watched the Portal light up with a swirling ring of rainbow light.
Finally.
They all rushed into the room as the light from the Portal died down and gravity returned to normal, Bill at the back, walking leisurely. Unlike the others, he knew exactly what was going on.
And there, still weak from interdimensional travel, was his old pal, Stanford Filbrick Pines.
Smiling, Bill stepped forward, reaching into his vest as he did.
"Well, that little charade took long enough," he said, an unnatural grin splitting his face.
"Bill—" Ford said, reaching towards his waist for his gun.
Bill was quicker.
With an explosive blast, Bill fired his pilfered gun, and the Author of the Journals' black outfit was dripping red.
Before any of the others could react, Bill swung to them.
One pull on the trigger, and Soos fell lifeless to the ground.
Another, and Stan collapsed backwards, fighting spirit useless.
And finally, Shooting Star, with tears dancing in her eyes.
"How could you, Dipper?" she asked.
"I'm afraid you've got me mistaken with someone else," Bill smirked. The trigger was warm under his finger.
And Shooting Star fell.
With Pine Tree's screams echoing in his ears, Bill walked forward to the Portal. This thing hadn't lasted long, but it had been long enough. There were specks of interdimensional rift floating in the air, and Bill swept them together with the palm of his hand. Enough for him to use, easily.
Clumped together, the rift began to pull at the air around it, at the barrier between the human world and the Nightmare Realm. Bill watched as it picked up its pace, laughing as the reality around him tore itself to pieces, as the glowing red eyes and shifting forms of the Nightmare Realm's inhabitants faded into view.
Dipper was screaming, shaking his sister's body as it bled out, wailing like his world had been destroyed.
And in every sense of the word it had been.
Bill was laughing.
Everything had gone perfectly.