Shadoo

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

- Shadoo

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 copying each hero and their abilities.

Super Paper Mario
Its true form is never revealed and only appears behind the screen as a blinking eye upon its first meeting with the heroes, where it introduces itself (albeit without saying its name) and asks the Heroes to go through the pit a second time, then calling a warp pipe for them. Upon the second meeting with the heroes, it admits its evil intents and announces that their 'clones' are ready, before a dark version of Luigi jumps onto the battle field (although, it is actually mocking Mr. L) which starts the battle. Upon Dark Luigi's defeat, Dark Bowser will appear on the battlefield, then Dark Peach, and finally Dark Mario. Each form has 100 HP and an attack of ten, although damage done from Dark Mario's hammer, Dark Bowser breathing fire, and Dark Luigi's super Jump is doubled, causing 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 or half progress in the Pit will be lost if the player saved before reentering Flopside Pit of 100 trials.

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.

Identity
As with Mimi, Shadoo's true identity is shrouded in mystery. However, judging by its self-description, shapshifting powers, and mention of revenge against the Pixls, it may be possible that Shadoo is the Pixl Queen's incarnation. Also, Shadoo's speech bubbles are the same as Tiptron's (albeit without the rainbow colors) suggesting that it, too, is a robot.

Trivia

 * Shadoo's name is a combination of the words "shadow" and "duplicate".
 * 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.
 * Much like the fight with Mr. L, when Shadoo is about to heal with a Shroom Shake, the player can steal the shake by using Thoreau.
 * The way Shadoo appears before fighting is similar to how Doopliss appears after scanning Mario or his Partners in Paper Mario: The Thousand-Year Door.