Uchiha Clan Downfall

The Uchiha Massacre is an event that took place about 4 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 Sasuke Uchiha. They did it to scare little Sasuke into also getting Mangekyo Sharingan, so Itachi could eventually steal his eyes to get an eternal Mangekyo Sharingan like Madara did long ago.

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