Shadoo is the boss fought at the 100th floor of the Flopside Pit of 100 Trials. He is a mysterious shadow entity and doppelganger created by the Ancients.
Shadoo was locked at the bottom of the Flopside Pit of 100 Trials, and planned his revenge against the Ancients. When Mario, Peach, Bowser, and Luigi, the four heroes who saved the world from Count Bleck and Dimentio (although the player has the option of fighting Shadoo before completing the final level) reached Shadoo's floor, Shadoo, unseen, told them to complete the Pit of 100 Trials again. Once they did, Shadoo revealed that he did not work for the Ancients, and wanted to take their place as the four heroes and get his revenge. His reason for the delay was to transform into dark clones of the four heroes. First he took the form of Dark Luigi, whose shadow was really that of Mr. L, and has exactly the same fighting style. Once defeated, he took the form of Dark Bowser, a dark version of Bowser. Once this was defeated, he became a Dark Peach, and after this, Dark Mario. Once this form was defeated, Shadoo was destroyed. The heroes' reward for this was great; their attack power was doubled, their flip meter never went down, and they got catch cards for Mario, Peach, Bowser, Luigi, and their dark clones.
Each of his forms has 100 HP, and all have 10 ATK power (including bumping into him) except Dark Bowser, who has 20 ATK.
First, Shadoo takes the form of Dark Luigi. His fighting style is the same as Mr. L; he jumps around and will use the Super Jump. He will also try to use a Shroom Shake, which the player must jump on him to stop him, or else steal it with Thoreau. He can be defeated the same way, by jumping on him, preferably as someone besides Bowser, who is a bit of an easier target for someone faster and who can jump on spiked enemies (as all enemies in the game can).
Second, Shadoo takes the form of Dark Bowser. His fighting style is the same as the original Bowser's, except this time, the player can control Bowser (as it is not the real Bowser that they are fighting). Breathing fire as Bowser makes it nearly impossible for Dark Bowser to reach Bowser, as he can't jump far and often jumps into the fire.
Third, Shadoo takes the form of Dark Peach. This battle is trickier as the player has not fought Peach or any variation of her before in this game. She attacks by jumping, often using her parasol. She will also use this to guard against jump attacks. Barry is a good defense against her, and she also does not seem to defend against Pixl attacks or Bowser's fire.
Last, Shadoo takes the form of Dark Mario. He attacks by jumping and by using his hammer for 20 damage. Jumping on him is risky due to his hammer, but if the player gets far enough from him as Bowser, breathing fire is effective, as it is difficult for Dark Mario to jump over the fire. Sometimes if off the screen he may just stand still until the player gets close enough to him. Once Shadoo's Dark Mario form is defeated, he will finally fall.
- Shadoo's battle theme is the miniboss theme, and his death scene is the same as the regular enemies (either vanishing in a puff of smoke or falling off the screen, depending on what attack was used to finish him).