Negative World

The Negative World is an anomalous course reported in Super Mario 64 DS associated with the process of unlocking Waluigi in several reports. It usually appears as a gateway to the level where Waluigi can be found, typically the Piranha Pit, and most commonly appears in the white room on the Second Floor through a purple door with an upside down L unlocked after clearing a special version of Chief Chilly Challenge, or a painting hidden somewhere in Princess Peach's Castle. Similarly to the mirrored castle, it has been described as an inverted and flipped version of Princess Peach's Castle, hence the name.

Description
As previously stated, the Negative World is described as an inverted and flipped version of Princess Peach's Castle. It doesn't lead to any levels, but has all of the floors, and some have even claimed it has a 4th floor of some kind, though this is uncommon. The only known level to appear in the Negative World is a level unlocking Waluigi, most commonly the Piranha Pit. Notably, the Castle Grounds is replaced an entirely with a destroyed town with a purple sky, though this is also just as uncommon as the 4th floor's reports. There is also reportedly a unique version of Chief Chilly Challenge in the area's equivalent of the Mirror Room, though this is only slightly more common than the other rare appearances, and only appears if Chief Chilly Challenge was not a prerequisite for the Negative World..

There have been reports of unique entities in the Negative World. The most common of these is a translucent Waluigi seen running around the level, usually leading to the Waluigi painting. Another common one is inverted Toads, who usually speak in complete gibberish or reversed text of regular Toads. Oddly enough, some have claimed to see Waluigi plushies that function similar to rabbits, though it was never specified what happened if they were captured, so this was likely a hoax.

Theorizing
This area appears to have been initially cut alongside all other traces of Waluigi, and may have even been an entire new campaign considering it's a reskin of the entire castle, potentially even with new areas. Waluigi likely would have been the center of this new campaign, but it's unknown. Alternatively, this could be the Personalization A.I. attempting to create a new and exciting area to lead up to unlocking Waluigi, contrasting the path to every other character level. This is likely due to Waluigi's inclusion as an extra character in the game as opposed to the 4 main characters.