From PhoenixWillowsRox88: "Do you know the Pirate AU? Where everyone's a pirate except Mabel who was cursed to be a mermaid. Bill is the captain of 'The Mindscape' which is following the good ol' 'Mystery Shack' to find treasure, and he eventually kidnaps Dipper for reasons unknown. Anyway, could you please do one where it shows Bill kidnapping him and then how he puts Dipper in a cage and monologues his plans? Villains always do that..."

I do! (You listed two AU ideas so I'm going with the one I know better, sorry.) I really love this idea, especially since it's been so long since I've written a Bill & Dipper interaction. The plot hasn't been told entirely in the au yet so I'm going off on a limb here, sorry. Here goes nothing! (excuse my bad attempt at pirate lingo)

Only the Best

Grunkle Stan was going to kill him. Dipper rested his chin on his hand, glaring up at the tall blond leering over the cage he was in (Seriously? A cage? He couldn't even stand up in it, it was that short). He really wanted to get out of this thing so that he could sock that creep in the face. He didn't even know why Cipher had kidnapped him in the first place.

That last thought caused his glare to morph into a frown and he looked down, thinking hard.

Why had Cipher kidnapped him?

"What's the matter, Pine Tree? Cat got ye tongue?" the pirate teased, smirking at the younger. Ah, yes. There was also that weird nickname too.

"Why am I here?" Dipper asked, trying not to seem too curious. "You could have taken anyone on The Mystery Shack and you chose me? I hate to disappoint you if you're using me as ransom but - "

Cipher cackled, startling the boy. "Oh, that's a riot, kid! Ye think that I'm using ye as ransom?"

Dipper swallowed hard. "Well, I mean, I couldn't really think of any other reason why - "

The man laughed again, shaking his head. Turning away from the cage and walking a few steps toward the portholes, Cipher twirled a knife between his fingers. Dipper didn't know how he wasn't chopping off his fingers doing so.

"Pine Tree, ye mistake me for a more lenient man. See, I have known Stan Pines for many years and let's jus' say... ye are me insurance." He chuckled after that. Dipper didn't like the way the captain had said that.

"What?"

"There is a bigger scheme than kidnapping a little cabin boy, kid," Cipher said and glanced back at the brunette with a mocking smile. Was it just Dipper, or did the man's teeth look pointier than before?

He swallowed again, trying not to seem like he was bothered by this pirate's words. "What is that supposed to mean?"

"Revenge on Pines is in order," Cipher snarled, his expression changing in an instant to something frightening. He looked like a beast ready to pounce on its prey. The boy found himself pressing farther back into the cage. "That bilge-sucking fool cost me everything last time...it won't happen again. So it's time to turn the tides."

Dipper noticed that the pirate's words were more proper when he spoke this time. "I still don't understand why you need me then..."

Cipher stalked toward the cage and the boy's eyes widened as he could have sworn he spotted the man's eye flash red. "Well, what if something goes wrong? What if Stanford shows up with his little crew? I could be wrong, but I highly doubt your great-uncle would do anything to endanger you, Pine Tree."

He snickered at the shocked look on Dipper's face. "Oh, you thought I didn't know? Pine Tree, I know lots of things! Besides, given the way he reacted when he saw you were gone - ha! Old fool! - I'd say you two were fairly close."

Dipper sucked in a shaky breath. The captain was leaning down, tracing his finger along one of the steel bars of the cage. "You're wr-wrong. He probably wouldn't care if anything happened to me."

Cipher rolled his eyes, still smiling in a rather twisted way. "Oh, kid. You're a terrible liar."

"And you seem to have improved your vocabulary," Dipper shot back, hoping he sounded braver than he felt.

The words didn't seem to throw the blond. Instead, they seemed to make him laugh. Cipher wiped a mock tear away as he shook his head. "I gotta admit, you're smarter than I thought, kid! I think I've said too much, though. Wouldn't want to spoil the surprise, eh?"

As he spoke his one eye lit up with blue fire and Dipper paled, a small strangle sound escaping his lips as he backed away. Okay, he definitely wasn't imagining that!

"What are you?" he muttered, not intending the captain to hear him.

"Oh, Pine Tree," Cipher said, leaning closer much to the brunette's dismay, "haven't you ever seen a demon before?"

I'm so mean to poor Dipper. Also, I have a headcanon that since Bill is probably a demon in this au posing as a pirate (or a pirate with demonic powers, idk) he probably just uses pirate lingo so that none of his crew questions why he talks using proper English. I dunno.