The Mark on the Sage of Six Paths' Forehead: The mark on the forehead of the Sage of Six Paths is a mystery in the anime series Naruto. It appears when the Sage awakens the Rinnegan, suggesting a connection to his Six Paths power. Orochimaru, who gained Six Paths power, does not have the mark on his forehead like the Sage.
However, during the Fourth Great Ninja War, Orochimaru became the Jinchuriki of the Ten-Tails and absorbed the God Tree. His forehead then displayed the same Rinnegan as Kaguya, allowing him to use the Infinite Tsukuyomi. The mark on the Sage's forehead remains unknown in its purpose.
Some speculate it is an incomplete form of the Rinnegan. Only Kaguya and Orochimaru, after becoming the Ten-Tails Jinchuriki, have the red Rinnegan. The mark on Sage's forehead represents his connection to the Tailed Beasts, but becoming a true wielder of the Rinnegan seems to require additional steps.