"Within Hell" (地獄の中で, Jigoku no Naka de, Viz: Living a Nightmare) is chapter 400 of the original Naruto manga.
When the Uchiha clan began plotting to take control of Konoha, Itachi was tasked to spy on Konoha's leadership for the Uchiha. Having witnessed countless deaths in his childhood and knowing that a coup d'état would cause more death, Itachi instead spied on the Uchiha for Konoha. When it became clear that war could not be avoided, Itachi was ordered to wipe out his own family. Choosing the lesser of two evils, Itachi sought out Tobi's help with the Uchiha's assassination, an opportunity for him to get revenge on the Uchiha for abandoning him decades earlier. Itachi killed the entire clan, but could not bring himself to kill Sasuke too. He asked Konoha to keep Sasuke safe and gave Sasuke the goal of killing him to make up for what he had done.