Naruto Shippūden: Season 6, Episode 136

6 - 136: The Light & Dark of the Mangekyo Sharingan

Itachi reveals to Sasuke that Madara is the second culprit behind the Uchiha massacre, as well as the history of the Mangekyo Sharingan and the first two Uchiha to obtain it: Madara and his brother. One question still remains: Why did Itachi really spare Sasuke's life?