Complex sentence: Sasuke is one of the few surviving members of the Uchiha clan, an infamous and powerful extended family of ninjas who were allied to the village of Konohagakure, before being massacred by Sasuke's older brother, Itachi Uchiha, before the series' start.
Split into simpler sentences: Sasuke is one of the few surviving members of the Uchiha clan, an infamous and powerful extended family of ninjas. The Uchiha were allied to the village of Konohagakure but were massacred by Sasuke's older brother, Itachi Uchiha, before the series' start.