Shadoo is the boss of the Flopside Pit of 100 Trials in Super Paper Mario. Shadoo's name is a play on the words "shadow" and "duplicate". While it was created by the Ancients, it works for its "own cursed powers". When the four protagonists make it to Room 100 for the second time, it copies their abilities to produce and shadowy clones in order to defeat them and take their places as part of a bid to get revenge on the Ancients. Even if the player does the Pit before Luigi joins the team, Shadoo will still present clones for all four heroes; the Luigi clone will always be a silhouette of Mr. L instead of the regular Luigi.
With all four forms combined, Shadoo is the strongest enemy in the game (having 400 HP altogether), even stronger than the game's final boss, Super Dimentio. Also, much like the fight with Mr. L, when Shadoo is Dark Luigi, he can heal with a Shroom Shake, which the player can steal by using Thoreau.
Shadoo's true form is never revealed, and is only hinted at via a shutter effect upon its first meeting with the heroes. It introduces itself (albeit without saying its name) as a servant of the Ancients, sent to the bottom of the Pit to grant power to the Heroes of legend. However, the first time Mario and the others reach the bottom of the Pit, Shadoo makes a Warp Pipe appear and asks them to beat the Pit again in order to prove that they are the Heroes. Upon the second meeting with the heroes, Shadoo reveals that while it was created by the Ancients, it does not work for them, and was actually using the Pit to test the Heroes so that it could scan their abilities and make clones of them. Shadoo then says it planned to use the clones to defeat and replace the Heroes in order to use the Pixls to get revenge on the Ancients.
The first clone to appear is a dark version of Luigi. 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, the player is awarded with 9990 score points for each form (for a total of 39960 points), and 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. In the Japanese and European/Australian versions, Dark Luigi's Catch Card is Mr. L's silhouette, rather than Luigi's, in order to match the clone's appearance.
Shadoo's true identity and backstory are never revealed; however, it bears some resemblance to the Pixl Queen discussed by Carson. The Pixl Queen was created by an Ancient, and after developing a grudge against her people due to how they used the Pixls, she attempted to overthrow the Ancients by taking control of the Pixls, similar to how Shadoo hated the Ancients and wished to use the Heroes' Pixl companions to exact its revenge. The Pixl Queen was also said to have "powers of illusion", which she used to hide from sight, just as Shadoo hid itself, a Warp Pipe, and the clones. Furthermore, the details of the Pixl Queen's fate is not revealed, with Carson only stating that she was "defeated", with the Dark Prognosticus being "taken from her" after her defeat, suggesting she survived the battle (unlike the Pixl that helped defeat her). Additionally, its speech bubble resembles the computer-type bubble used by characters like Tiptron, Fracktail, and Wracktail, and mechanical sounds can very occasionally be heard while Shadoo is idle during dialogue.
Wracktail is implied to be familiar with Shadoo, as after being beaten by the heroes, Wracktail states that it "wasn't the only monster created by the ancients" and directs them to the Flopside Pit of 100 Trials.
Names in other languages