Big Boo Battle
- “Big Boo's Haunt Eee hee hee... There are no exits. You may wander in here forever... Listen carefully. Can you hear me? If you find me, you're history. Eee hee hee...”
- —King Boo
Big Boo Battle is an additional level exclusive to Super Mario 64 DS. The entrance to the level is found through the Luigi portrait in Big Boo's Haunt's third floor. The level is accessible to anyone other than Yoshi, as Yoshi cannot defeat the Big Boo in the castle's courtyard to enter Big Boo's Haunt. After defeating King Boo (who is erroneously called "Big Boo" in the American releases) at the end of the level, the player will be rewarded the Luigi Key to unlock Luigi. Initially, only Mario can enter the course, as Luigi is needed to unlock Wario. It is one of the two secret levels in Super Mario 64 DS to be inside a main level; the other being Behind the Waterfall in Hazy Maze Cave.
Layout
Throughout the course, the player can hear King Boo's laugh echoing through the rooms. The rooms are set up like a maze, and if the player goes through the wrong door or falls down a hole, they will be returned to the starting room.
The player will start of in a circular room with a Boo, a rotating pyramid in the center, and a rotating platform around the walls of the room. Taking the door on the right will lead to a square room with falling bridges and a Mr. I in the center. The room to the north has a large tilting platform with a ? Block over top of it, which will allow Mario to collect the Coins above with the Power Flower inside. Additionally, a Luigi Cap can be found in this room if he is unlocked. The room to the east has a large pit with lifts that will help the player move across the platforms; a Wario Cap can also be found in this room if he is unlocked. The final room to the north is a large hall with a falling bridge in the center. At the end of the bridge are several lifts that will help the player over the wall and to the hole leading to the arena with King Boo.
King Boo Battle
Upon falling into the hole, the player will be brought to a room with a large mirror. Approaching the mirror and pressing 
to look into it, the reflection of the player will turn into Luigi, though nothing will happen if Luigi is already rescued. After the reflection change, King Boo will appear next to the player and begin a short dialogue. Once the dialogue is over, King Boo will begin attacking.
At first, King Boo will only be visible on the other side of the mirror, though a large shadow will be visible on the player's side. Using this, the player can sneak up behind King Boo and attack. After the first hit, King Boo will become visible on the player's side, and then will be visible on the reflection's side once more after the second hit. After attacking King Boo three times, he will be defeated and drop the Luigi Key.
King Boo's methods of attacks are simply running into the player or shooting a blue fireball, the latter of which will only happen if King Boo is on the player's side of the mirror. Like with other Boos, King Boo will turn invisible and will become unable to be attacked if the player character looks at him. If the player doesn't attack King Boo quick enough, he will teleport to the side of the mirror opposite of the one he is currently on. When on the player's side of the mirror, he will not be reflected in the mirror nor will a shadow appear on the opposite side. Additionally, King Boo moves faster with each phase. If the player needs to recover damage, there are rows of Coins along the walls.
Similar to the large mirror in Peach's Castle, the Camera Lakitu can be seen in the mirror's reflection.
Missions
There are two Power Stars the player can collect in this level.
Star 1
Located throughout the level are eight Red Coins. In order to collect them all, the player needs to have the Power Flower and Wario unlocked. Once all the Red Coins are collected, the Power Star will appear at the Star Marker in the large hall with the falling bridge. The location of the Red Coins are:
- In the first room, opposite of the player's starting position.
- Two are in the next room: one on the platform with the Mr. I and one over the falling bridge to the east.
- Two are in the next room: one on the platform to the east and one in the air in the back-right corner. Mario's Power Flower ability is needed to collect the latter.
- Two are in the next room, both of which require the use of the lifts to collect.
- The final Red Coin is under the Black Brick in the next room, which only Wario can destroy.
Star 2
There is a hidden Star behind the Boo portrait in the last room before the boss. To collect it, the player needs to go to the room with the ? Block and grab the Luigi Cap if they aren't already playing as Luigi. As Luigi, the player needs to grab the Power Flower from the ? Block, which will turn them invisible. Before the power-up wears off, the player needs to quickly move through the rooms and jump through the large Boo portrait after the falling bridge to collect the Power Star. To leave the small room, the player can simply walk though the portrait again.
