Power Ztar

Power Ztars, also known as Shadow Stars, are anomalous collectibles that appear in some personalized copies of Super Mario 64. They take the appearance of recolors of power stars, being black with purple eyes instead of yellow with black eyes. They can appear in place of a power star, with the replaced power star usually being relocated or removed entirely. If the Power Ztar is collected, it can reportedly damage the player or decrease the power star count by one. The lost star would then need to be recollected, though the lost star can supposedly be lost forever.

Power Ztars are usually reported by players that perform well in the game. This appears to mean the Power Ztars are created by the Personalization A.I. in order to increase the game's difficulty, though some have reported a second form of manifestation of the Power Ztar anomaly that does not support this theory.

The second main manifestation of Power Ztars are side collectibles that, when enough are collected, can usually unlock a special level. This level usually does not appear as a full course, instead being a smaller level with 1 or 2 stars. Notably, some reports claim this level can be a Bowser level, though these are unconfirmed. This manifestation of Power Ztars appear to be made in order to give the player more content in the game, and make the game more enjoyable.

Trivia

 * Power Ztars actually appear in Mario Party, taking away a power star similarly to it's first main manifestation in Super Mario 64. This appears to imply that the Power Ztar was reused in Mario Party after being scrapped from Super Mario 64, or the collectible is basic enough to coincidentally be recreated by the Personalization A.I.