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 on 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 Mushroom Castle's courtyard to enter Big Boo's Haunt. After defeating King Boo (who is erroneously called "Big Boo" in the English version) at the end of the level, the player is rewarded with 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, and the only level within another level in the game where another playable character can be rescued.

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 are returned to the starting room.

The player starts in a circular room with a Boo, a rotating pyramid in the center, and a rotating platform around the walls of the room (similar to Big Boo's merry-go-round). Taking the door on the right leads 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 the top of it, which allows the player 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 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 help the player over the wall and to the hole leading to the arena with King Boo.

Boss battle
Upon falling into the hole, the player is brought to a room with a large mirror. By approaching the mirror and pressing to look into it, the reflection of the player turns into Luigi, though nothing happens if Luigi is already rescued. After the reflection changes, King Boo appears next to the player and begins a short dialogue. Once the dialogue is over, King Boo begins attacking.

At first, King Boo is visible only on the other side of the mirror, though a large shadow is visible on the player's side. Using this, the player can sneak up behind King Boo and attack. After the first hit, King Boo becomes visible on the player's side, and then he becomes visible on the reflection's side after the second hit. After the player attacks King Boo three times, he is defeated and drops the Luigi Key.

King Boo's methods of attacks are simply running into the player or shooting a blue fireball, the latter of which happens only if King Boo is on the player's side of the mirror. Like other Boos, King Boo turns invisible and becomes unable to be attacked if the player character looks at him. If the player does not attack King Boo quickly enough, he teleports to the side of the mirror opposite of the one he is currently on. When on the player's side of the mirror, he is not reflected in the mirror, nor does a shadow appear on the opposite side. Additionally, King Boo moves faster with each phase. There are rows of coins along the walls, letting the player recover damage.

Similar to in the large mirror in the Mushroom Castle, the Lakitu Bro 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 appears at the Star Marker in the large hall with the falling bridge. The locations 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. The one in the air can be accessed either with Balloon Mario or with a triple jump performed by either Mario or Luigi.
 * 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 are not already playing as Luigi. As Luigi, the player needs to grab the Power Flower from the ? Block, which turns 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 through the portrait again.

Trivia

 * In the final room of the level, there are three Boos that, unlike the other Boos throughout the level, can respawn after they are killed, as well as retaining their coins. If the player were to fall into a pit or enter one of the wrong doors in the maze, they would be able to collect the same coins again. This means that there are technically an infinite number of coins to be collected within the level.
 * The maze doors in this level are similar to Lost Woods in The Legend of Zelda: Ocarina of Time, as both are series of four-way paths with wrong turns leading to the entrance, and the player should identify the correct paths by finding the side with the loudest sound.

Dimora di Re Boo