Black Bowser's Castle is the final level in Paper Mario: Color Splash. Mario and Huey are taken here by Luigi in his kart along the rainbow road created by the Big Paint Stars.
The castle is full of long hallways, as seen in the beginning of the level. When Mario enters the castle, a paintless Princess Peach is hung on the wall. When Mario goes up to examine it, a trap door opens and Mario falls into a small cell, where a battle with Roy Koopa is initiated. Once Roy is defeated, Mario will enter a factory with a bunch of Hammer Bros. and Paint Guys. Eventually, Black Paint starts flooding the castle, and Mario must make his way back up to Black Bowser's throne room. Mario fights Black Bowser, and flees with Peach, meanwhile Huey risks his life to ensure the black paint would not harm anyone else. After these events, Black Bowser's Castle becomes a card that can be used in battle.