Eternal Fort

The Eternal Fort is an elusive and mysterious course from Super Mario 64 spread by rumour and word of mouth. The existence of this area in the final game is heavily debated, and the few that insist that they've stumbled across it either refuse to divulge details about the level itself or go dead silent after saying so.

Information
Information on the Eternal Fort is scarce, with no coherent image or depiction of the level itself. What is known is that the Eternal Fort was intended to be the ultimate and final level in Super Mario 64, but was removed before the final build for unknown reasons. It's also unknown if this level was intended to be in place of Bowser in the Sky, or as a bonus level after beating the game.

A video surfaced online claiming to be the extracted music associated with the Eternal Fort, but viewing of this video could potentially be dangerous as hinted at by the description; Nintendo appears to be keeping this level under wraps for an unknown reason.

There are very minimal credible sources that refer to Eternal Fort, and even the ones that do are often very vague, in passing or go radio silent after referring to it. The ones that do speak up usually only have years after, and either denounce the game entirely or appear utterly distressed over whatever they came across.

Speculation
Given by the words that Nintendissident and the King of the Internet have said about the Eternal Fort, it appears that Nintendo is intentionally keeping the existence of the level under wraps and down low. The reason why is unknown, but the two users discuss the idea of a "final truth" hidden within the level.

The Eternal Fort's existence is tied to the Personalisation A.I., where many believe people will only find it if they find out everything there is to know about Super Mario 64. On an alternative and more innocent note, the level could've proven too difficult and Nintendo removed it due to its difficulty. Therefore, the A.I. only summons the level for experienced players who have proven themselves worthy of playing the level.

The Eternal Fort may also be tied to the similarly named Eternal Star Statue as seen within the Castle Courtyard. Not only do the two share similar names, but the King of the Internet refers to the infamous L is Real legend in his comment. With that, it can be assumed the Eternal Star Statue is related to or a remnant of the Eternal Fort, or that Luigi is associated with the Eternal Fort.