Chief Chilly Challenge

Chief Chilly Challenge is a level in Super Mario 64 DS found behind the mirror in the Second Floor's mirror room. This level is only accessible with Luigi, as his Power Flower ability, the Vanish Cap, allows him to pass through the mirror. This level's boss is Chief Chilly, and beating them unlocks Wario as a playable character.

Geography
Chief Chilly Challenge begins with a small platform connected to a slide leading to the rest of the level. It begins with another platform, a series of tilting platforms, and then another platform that moves between 2 different platforms. This then leads to a platform breaking off into a gust of wind next to several coins. This then leads to a platform with water, another platform, and then more platforms eventually leading to the Chief Chilly boss fight. The only enemies in this level are Bullies.

Stars
There are 2 stars in this level. One is obtained from collecting 8 red coins, and another is found in a black brick block that requires Wario to break. The black brick is past a path with wind that will blow the player off of the map, so Wario's Power Flower ability, the Metal Cap, is also required.

Super Mario 64 Manifestation
In some personalized copies of Super Mario 64, Chief Chilly Challenge can appear as a level. Notably, in place of the regular Bullies, it has been reported that the unused small Chill Bullies can appear in the level.

Hidden Mission
Chief Chilly Challenge can reportedly have a "secret mission" involved with unlocking Waluigi. You first have to collect the Power Flowers in the mirror room in an upside-down L formation, and then enter the level. It will be mildly different. You then have to collect all coins in the level, including red coins, totaling up to 79 coins total. Then, enter the Chief Chilly boss fight, and hit them to make them drop coins. Upon hitting 100 coins, a Mushroom will appear, allowing you to become huge. You can use this to defeat Chief Chilly easily. After defeating Chief Chilly, players have reported the entrance to the Negative World appearing in the white room, or a Waluigi painting appearing somewhere in the game, usually leading to the Piranha Pit. Alternatively, Chief Chilly can then drop Waluigi's key immediately.