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. Shadoo's name is a play on the words "shadow" and "duplicate". It might also be a pun on the word "ado".

Super Paper Mario
Shadoo's 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.

Upon Shadoo's 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 the third dimension infinitely.

Trivia
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 * In the NTSC-J and PAL version, Dark Luigi's Catch Card more closely resembles Mr. L, to match its appearance when the player fights it.
 * 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.
 * Despite being at the bottom of the Pit of 100 Trials, Shadoo's battle theme song is the same as the one that plays when a miniboss is being fought. In addition, the way his forms die is identical to that of any regular enemy.
 * With all four forms combined, Shadoo is the strongest enemy in the game (having 400 HP altogether!), even stronger then Super Dimentio, similar to how The Master and Bonetail are the strongest in the game, even more so then the final boss, from Paper Mario and Paper Mario: The Thousand-Year Door, respectively.