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 he was created by the Ancients, he works for his "own cursed powers". When the four heroes make it to Room 100, he tries to beat them and take their place to take revenge on the Ancients with the Pixls by turning into each hero and copying their moves.

He'll first turn into an entirely black form of Luigi, then Bowser, Peach, and finally Mario. Each form has 100 HP and an attack of 10, and when the form of Mario uses his hammer, 20. 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 and all progress in the Pit will be lost.

Upon defeat 8 catch cards appear: one of each of the heroes and their dark forms. Additionally, Mario's 3D flip meter will never run out.

Trivia

 * The first time the heroes make it to the bottom, Shadoo is hidden and says that he is there "to give power to the heroes". But he is not sure if the four really are the heroes, so he challenges them to make it through the Pit again, which leads to what is described above.