Shadoo is one of the optional superbosses in Super Paper Mario, the other being, Wracktail. He resides at the very bottom of the Flopside Pit of 100 Trials claiming to be created by The Ancients. He makes the heroes go the pit twice before getting the chance to fight him. His true form is never revealed and is only seen through a "blinking eye" in the background and fought through shadow clones of the heroes. First of which is Dark Luigi (similar moveset to Mr. L), Dark Bowser (similar moveset to Bowser), then Dark Peach who attacks by hovering with her parasol and jumping on the player, then lastly fights as Dark Mario who jumps and attacks the hammer (in a similar fashion to the Cudge Pixl) dealing heavy damage. Once the player defeats him in all forms, 8 treasure chests open up with 8 cards representing the regular and dark versions of each hero in addition to Mario's Flip Ability upgraded to work infinitely.
- Based on Carson's stories, it is heavily implied to be the villainous Pixl Queen reborn in a new form as noted from their mention of being created by The Ancients but not working for them as well as stating their intentions to supplant the 4 heroes right before engaging them.