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 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, sending the heroes back to the top of the Pit of 100 trials.
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 three dimensions forever.
- Transformation: Shadoo uses his transforming abilities to become Mario, Luigi, Peach, or Bowser and use their abilities as well like Mario's Stomping, Peach's Parasol, Bowser's Flame Breath, or Luigi's High Jump.
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 reincarnation. Also, Shadoo's speech bubbles are the same as Tiptron's (albeit without the rainbow colors) suggesting that it, too, is a robot.
- Shadoo's name is a combination of the words "shadow" and "duplicate".
- In the 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 its forms die is identical to that of any regular enemy.
- His powers are much like the Duplighosts from the original Paper Mario and Doopliss (who, himself, is a Duplighost) from Paper Mario: The Thousand-Year Door.