Within Darkness…!! (闇の中…!!, Yami no naka…!!) is chapter 225 of the Naruto manga.
Itachi confirmed that he had killed Shisui to acquire the Mangekyō Sharingan, the death of one's closest friend being needed to awaken it. He directed Sasuke to the Naka Shrine and encouraged him to use the information stored there to become enough to gain power and kill him, avenging their clan. Sasuke passed out and woke up as the last Uchiha in Konoha. Distraught over the loss of the family that had just started to accept him, Sasuke agreed to live the rest of his life hating Itachi. Sasuke, realising that Naruto his his closest friend, decides to kill him to pave the way towards killing Itachi.