Shadoo is an evil being who challenges everyone he meets with dark versions of that person, and is seen only in Super Paper Mario. He is located in the 100th room of the FlopsidePit of 100 Trials. In order to face him you have to do the Pit of 100 Trials twice. The second time you enter the room he will confront you as a dark version of Luigi, Mario, Bowser, and Peach. All of these forms have 100 HP, and can deal 10 damage (20 if he uses Bowser's fire breath or Mario's hammer). They are all fought one at a time, and the player never battle the real Shadoo. Shadoo was created by the Tribe of the Ancients, like Fracktail and Wracktail. His real form is not seen in the game, as only his speech bubbles appear along with the clones. The rewards are 9990 EXP for each form of Shadoo, and cards of the main characters, which double attack power. As for Mario's card, you can stay in 3D as long as you like without the 3D meter depleting.
It is highly recommended to level up often and have only potent recovery items in your inventory, as well as collecting cards of shadow versions of enemies when going in the Pit of 100 Trials to beat Shadoo.
Shadoo may be Dimentio's "last surprise", but it isn't confirmed to be so, and the Flopside Pit of 100 Trials can be done before you finish the game (although it is strongly recommended that you do beat the game first).
His first form is actually Mr. L, although it looks like Luigi at first (you can see Mr. L's bandana on Dark Luigi's neck).