King Bill

King Bills are a brand new sub-species of Bullet Bill, which only appears in the last level of New Super Mario Bros. Wii; World 9-8.

It's very large in size, being bigger than even the Banzai Bills. It is the biggest enemy of the game, though it's not bigger than Bowser's giant form after Kamek's spell in Bowser's Castle. King Bills are also invincible, meaning that they can only be defeated by jumping on the flagpole at the end of the level.

There's a total of 10 King Bills in the entire game.