The Light & Dark of the Mangekyo Sharingan

As Zetsu watches on, Itachi explains to Sasuke the secret behind the Mangekyo Sharingan, and the history of the Uchiha clan. Besides being able to tame the Nine-Tailed Fox, the Mangekyo will eventually rob the user of his eyesight and the only way to overcome this obstacle is to steal the Sharingan eyes of that person's sibling. Sasuke retaliates against Itachi's assaults, but he is eventually pinned to a wall and has one of his eyes seemingly stolen by Itachi.