Luigi's Casino

Luigi's Casino is an area seen in several minigames of Super Mario 64 DS, run by Luigi, who has a unique outfit on. A Toad in a business suit also appears in the background occasionally, carrying drinks, presumably to some guests. You can see a chandelier, arcade machines, and what might be a pool table. Some people have attempted to find it in the game, leading to it's discovery as an anomalous level.

Unlocking
The level is usually described as an anomalous course only playable by Luigi. How it is found is inconsistent in reports, likely due to personalization, but it typically appears behind the unused 5-star door found in the files of SM64DS, and usually requires the player to beat all of the Luigi's Casino minigames before the level is unlocked. Alternatively, it has also been said to be unlocked by the unused Big Star Key, likely due to the area's association with minigames as the regular Star Keys dropped by rabbits unlock minigames.

Geography
Luigi's Casino is typically described as an expanded version of the area seen in the minigames, with more areas typical of a casino, such as other games to play or even a bar of some kind where the Toad can be seen. If Luigi is played in this level, some have claimed that they use the suit seen in the minigames, or can appear as an NPC if they aren't played as. Some have even claimed some objects are interactable, opening minigames, potentially how the Luigi's Casino minigames would have been played. No enemies are typically present, but NPCs such as Toads and Koopas can be seen around the level in most descriptions.

Missions
The most common missions are either finding 5 Silver Stars or 8 red coins, as well as stars obtained from playing the minigames that can appear in the level. Notably, a common mission reported involves a rematch with a Big Boo or even King Boo somewhere in the level, likely due to their associations with Luigi due to the Luigi's Mansion series, or King Boo's appearance in a casino in Super Mario Sunshine. Some other missions reported are obtaining one from talking to a Toad just like some stars obtained in Princess Peach's Castle, finding the star on top of a chandelier, or finding a hidden room behind one of the arcade machines in the level.

Theorizing
This level might have been scrapped from Super Mario 64 DS due to it's association with casinos, alcohol and gambling, which Nintendo might have had second guesses about due to their target audience being children, removing the level and restricting it's appearance to minigames. However, if this was the case, they likely would have removed it entirely. It also might have just been removed to round the Power Star count to 150.