The reason behind the Uchiha Clan Downfall. Is it a mission? Know why.

In a single night, Itachi and Tobi slaughtered the entire Uchiha clan. In the anime, Tobi slaughtered the military portion of the Uchiha Clan by directly attacking the Konoha Military Police Force headquarters, enabling Itachi to slaughter the civilian portion without much resistance. In the anime, Tobi secretly took some of the Uchiha's corpses for himself. Itachi expected Fugaku to be the greatest enemy and entered his home with great caution. To his complete surprise, both his parents were willing to accept death rather than fight their son. Both respected his decision to side with the village over the clan, and asked only that he look after Sasuke. He tearfully killed them just before Sasuke returned home from the Academy.

Although he knew his actions were necessary, Itachi could not forgive himself for killing his own family and believed he had to be punished. Sasuke, a child and thus ignorant of the Uchiha's conspiracies, was the only person qualified to pass the judgement on Itachi he felt he deserved. As such, Itachi attacked Sasuke when he entered their house, desperate to make Sasuke believe he had killed their clan without provocation and for selfish pursuits of power. He used Tsukuyomi to torment Sasuke with images of the clan's murder and claimed Sasuke was too weak to even bother killing. Itachi encouraged Sasuke to get stronger and to come after him to avenge their family.