Phantasm Cap

The Phantasm Cap is an anomalous version or upgrade of the Vanish Cap, similarly to other anomalies such as Gold Mario and the P-Wing Cap, found in some personalized copies of Super Mario 64. It has been described as having similar properties to the Boo Mushroom seen in the Super Mario Galaxy series, potentially being inspiration for this later power-up. It turns the player invisible, and also makes them entirely white.

Functionality
The Phantasm Cap reportedly maintains all previous functionalities of the Vanish Cap, making the player invisible, and also entirely white, allowing them to pass through certain barriers and sneak past enemies that can no longer see them. The Phantasm Cap can pass through even more objects, and also allow the player to temporarily levitate to reach certain areas. It has even on occasion been reported to make the player invincible to most enemies, through some reports claim the player is damaged by light, similarly to the Boo Mushroom.

Unlocking
The Phantasm Cap's method of being unlocked has commonly been associated with the Ghoul Medal, commonly requiring it to be obtained before the Phantasm Cap's switch can appear, likely due to the association with ghosts and Boos. Upon obtaining the Ghoul Medal, if it is necessary, there have been 3 reported ways of obtaining the Phantasm Cap. The first way is returning to Vanish Cap Under the Moat, where the Phantasm Cap Switch can appear in part of the level, occasionally replacing the Vanish Cap Switch. The second way is finding a new version of Vanish Cap Under the Moat somewhere in the game, though it's exact location is inconsistent. This new level would contain the Phantasm Cap Switch. The third and final way is finding a secret area in the basement of Big Boo's Haunt, reportedly resembling Big Boo Battle from Super Mario 64 DS, with the Phantasm Cap Switch being at the end of the level.

Notably, some reports claim that the Ghoul Medal is unlocked after the Phantasm Cap instead of it being the other way around, requiring the usage of the Phantasm Cap to reach the area containing the Ghoul Medal, wherever the location is in the copy, such as passing through the walls surrounding Big Boo's Haunt or walking through a wall in Vanish Cap Under the Moat or the special Phantasm Cap level if it exists.