[quote=@Roland]
I'd have an idea to use either a mage or a mage as the antagonist,
[/quote]

Excellent choices.

Well, if we're going for absurd, maybe the tavern is a living being and travels around just because it can. Maybe the barkeep and patrons know this, and they use it to go exploring. Maybe the barkeep uses as a test for local adventurers, and those who survive get hired to do adventurer things for him.

Or, depending on how much magic you like, maybe there's some weird planar influence and the tavern just so happened to get sucked into a different plane, and now you have to find your way back?

There's the old fashioned "barkeep was an evil wizard who works for a demon" approach and you guys were meant to be sacrifices, but the spell sent the whole ass tavern and now you're in hell, and you have to survive there.

There's endless possibilities. Maybe don't have a plan and fly by the seat of your pants. Your players can't know what you're planning If you don't have a plan.