Just before this Hero enters the first Room of your dungeon, change the order of your Rooms so the last (rightmost) Room becomes the first (leftmost) Room, the second-to-last becomes the second, and so on. This change is permanent. As with all effects that move Rooms, any “Room stacks” remain intact for the move.
The Boss ability can target ANY face-up item in play, including other player's scorekeeping area, Items in town, or an Item attached to a Hero.
This scene is reminiscent of Final Fantasy IV when Cecil Harvey becomes a Paladin in the secret chamber on the Mount of Ordeals and a shade of himself as a Dark Knight steps out of a magic mirror, sword drawn, to battle him.