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 copies their abilities and uses shadowy clones to try to defeat them and take their places, in a bid to get revenge on the Ancients. Shadoo's name is a play on the words shadow and duplicate; it may also be a pun on the words ado and doom.
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 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).
Shadoo's true form is never revealed, and only appears behind the screen as a blinking eye 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 (however, even if the player does the Pit before Luigi or Bowser join the team, Shadoo will still present clones for all four heroes). The Luigi clone, however, has Mr. L's bandanna. 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 9999 score points, and eight Catch Cards appear: one of each of the heroes and their dark forms. In the the NTSC-J and PAL version, Dark Luigi's Catch Card is Mr. L's silhouette, rather than plain Luigi's, to match the clone's appearance. 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.
It's Mario's dark-power doppelganger! One of the last foes in the Flopside Pit of 100 Trials.
That's Dark Mario. This fake Mario lives in the Flopside Pit of 100 Trials... Max HP is 100. Attack is 10. He has high attack power and jumps and hammers well... If you don't plan your stomp, he might hit you with his hammer. You share weaknesses, so if you can exploit that, you'll win... After all, he's not the real Mario... He's just a bad copy...
It's a dark duplicate of Luigi! One of the last foes in the Flopside Pit of 100 Trials.
That's Dark Luigi. He lurks in the Flopside Pit of 100 Trials... Max HP is 100. Attack is 10. He has the same incredible jumping ability as Luigi. Watch out for that super jump of his when you go to stomp him. But his jump could be an interesting opportunity for an attack as well... After all, he's not the real Luigi...He's just a bad copy...
It's a dark duplicate of Peach! One of the last foes in the Flopside Pit of 100 Trials.
That's Dark Peach, a phony Peach that dwells in the Flopside Pit of 100 Trials... Max HP is 100. Attack is 10. This Peach uses her parasol just like the original. And if she uses it in midair, she might mess up the timing of your attack... But that might also give you an opportunity to attack... After all, she's not the real Peach... She's just a bad copy...
It's a dark duplicate of Bowser! One of the last foes in the Flopside Pit of 100 Trials.
That's Dark Bowser. This fake Bowser rules the Flopside Pit of 100 Trials... Max HP is 100. Attack is 20. Defense is 4. He has spikes, so you can't stomp on him... He has high defense, so you'll need to use Boomer and Cudge... Watch out for his ferocious fire attack... But after all, he's not the real Bowser... He's just a bad copy...