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 Sharingan, but for twisted reasons, they spared Itachi's younger brother, Sasuke Uchiha. The reason for them killing their entire family, at least Itachi's was to make Sasuke hate him and want to kill Itachi. This would eventually lead to him trying to gain the mangeyko sharingan for power, which is exactly what Itachi wanted. This would allow him to steal Sasuke's eyes to gain the perfect mangekyo sharingan, just like Madara Uchiha did.

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