Shadoo

Mario confronts Shadoo.

“Oh heroes... Blessed souls..... I must destroy you all with the power of shadow!”

—Shadoo, Super Paper Mario

Shadoo is the boss of the Flopside Pit of 100 Trials the second time the four heroes reach Room 100 in Super Paper Mario. While it was created by the Ancients, it works for its "own cursed powers". When the four heroes make it to Room 100, it tries to defeat them and take their place to take vengeance on the Ancients with the Pixls by turning into each hero and copying their moves.

It's true form is never revealed for it first appeared as an entirely black form of Luigi (although, it is actually mocking Mr. L), then Bowser, Peach, and finally Mario. Each form has 100 HP and an attack of ten, although the form of Dark Mario using the hammer, Dark Bowser breathing fire, and Dark Luigi performing a super Jump inflicts twenty damage. If the heroes don't fight back smartly (such as neglecting to use Barry or even Peach's umbrella to deflect damage) the fight can be lost very easily and all progress in the Pit will be lost.

Upon defeat eight Catch Cards appear: one of each of the heroes and their dark forms. The hero cards will double the attack power of character on it and the Mario card will also allow him to remain in three dimensions forever.

Trivia

• The first time the heroes make it to the bottom, Shadoo is hidden and says that he/she is there "to give power to the heroes". But it is not sure if the four really are the heroes, so it challenges them to make it through the Pit again, which leads to what is described above.
• Judging by its self-description, shapshifting powers, and mention of revenge against the Pixls, it may be possible that Shadoo is the Pixl Queen reincarnated.
• In the PAL version, Dark Luigi's Catch Card more closely resembles Mr. L, to match Shadoo's appearance when the player fights him/her.
• Shadoo bares a minor resemblance to the Anuboo from Mario & Luigi: Superstar Saga, even being able to transform into Mario, Luigi, Princess Peach and Bowser, in the same way the Anuboo could transform into Mario and Luigi.
• Shadoo's speech bubbles are the same as Tiptron's (albeit without the rainbow colors) suggesting that it too is a robot.
• Dark Luigi and Dark Bowser fights the exact same way Mr.L and Bowser fights before joining Mario.