Uchiha Clan Downfall

The Uchiha Clan Massacre is an event that took place 5 years years prior to the Naruto plot. Itachi Uchiha and Madara Uchiha killed their entire clan in a single night using their Mangekyo Sharingans. For previously unknown reasons, they spared Itachi's younger brother, Sasuke Uchiha. Later in the manga, it is revealed that Itachi's reason for killing his entire family was to make Sasuke hate him and want to kill Itachi. This would eventually lead Sasuke to try and gain the mangeyko sharingan for power, which is exactly what Itachi wanted. By doing so, Itachi would take Sasuke's mangekyo eyes and replace them with his own failing ones, just like Madara Uchiha did.

At this time, the Mangekyo Sharingan is introduced to the Naruto series.