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 may also be a pun on the words ado and doom.

History
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 reveals its intentions and announces that their "clones" are ready, before a dark version of Luigi jumps onto the battlefield (although it is actually a shadow of 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, all of Dark Bowser's attacks, 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 the character on it, and the Mario card will also allow him to remain in the third dimension indefinitely.

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 weak bosses, such as Mr. L and the Big Blooper are 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 than Super Dimentio, similar to how The Master and Bonetail are the strongest in their respective games (Paper Mario and Paper Mario: The Thousand-Year Door), even more so than the final bosses, Bowser and the Shadow Queen, respectively.
 * It is possible Shadoo is some kind of revived form of the so-called "Pixl Queen" because Shadoo mentions have once being affaliated with, also created by the Tribe of Ancients and desire to take control of the heros' Pixls.