Eternal Star

The Eternal Star is a rumored collectible in Super Mario 64, believed to be a large Power Star. It is believed to exist from the engraving on the Castle Courtyard statue's plaque, which some have interpreted as "Eternal Star". Other theories believe that the Eternal Star is the large star seen in the ending after defeating Bowser in Bowser in the Sky.

In heavily personalized copies of the game, the "Eternal Star" may manifest as a collectible, most often associated with the Star Statue in the Castle Courtyard. The Statue in question is also referred to as the Eternal Star in some copies, making a common belief that they are one in the same.

Some people propose that the Eternal Star is the supposed 121st Star, or a method to unlock Luigi, the latter likely being due to the other common interpretation of the plaque being "L is Real 2401". This varies between copies, however.

Collecting the Eternal Star
The Eternal Star can supposedly be found in Super Mario 64, often found after the game is completed. This is not true in standard or emulated copies, however.

In personalized copies that feature the Star, It is usually reported to appear in place of the star seen on the Castle Courtyard statue, often after all 120 stars are collected. Some also report a "quest" of some kind relating to the Eternal Star, most often involving a playable Luigi, and usually some challenge of immense difficulty; clearing an exceptionally difficult personalized course, collecting every other star in the copy, or collecting a set of collectibles that can be used on the Star.

Reports are highly inconsistent, however reports of Luigi being a factor in collecting the Eternal Star are relatively common, making the two heavily connected.

Related Anomalies
The Eternal Fort is theorized to be in some way connected to the Star, due to their sharing of the word "Eternal". Theories that relate the two often believe that collecting the Eternal Star is in some way related to accessing the Fort, as either the primary or potential main unlock route. Theories linking the two are currently impossible to conclude on, since there have been no credible reports of a player accessing the Fort, and the few people that have claimed to see it refusing to disclose information.

Hard Mode is notable for, in every copy that currently features it, entirely locking out the Castle Courtyard, including the Eternal Star. A leading theory is that the Courtyard contains some final reward, perhaps a course, that directly results in the player obtaining the Eternal Star. Other theories attempt to link Hard Mode and Eternal Fort, claiming that completing Hard Mode may allow players into the Courtyard, where the Star grants players access to the Eternal Fort. This is also hard to conclude on, for the same reasons as above.