|Game(s)||Newer Super Mario Bros. DS|
|Appearances||(DS) The House of Stairs|
King Boo is a secret boss fought for The House of Stairs' secret exit in Moonview Glacier in Newer Super Mario Bros. DS, the only path to Lunar Realm through World 5-. He is, as his name implies, the king of Boos.
The player will enter King Boo's arena, which is a long hallway full of Circling Boo Buddies and Splunkins (along with their big and mega counterparts). Several Question Blocks and Brick Blocks can also be found, some providing powerups. Some spikes and lifts attempt to impede the players progress as well.
King Boo will slowly chase the player in an autoscrolling fashion, occasionally speeding up for a bit after laughing. He will also shoot ghostly balls at the player which zigzag as they hit the floor and ceiling.
At the end of the hallway is a ? Switch. Once the player reaches it, they must press it in order to turn on the lights, shrinking and defeating King Boo in the process.