Complex sentence: He ruled Tmutarakan under the overal authority of his father Sviatoslav Iaroslavich, Prince of Chernigov, but he was twice expelled from his principality by one of his cousins Rostislav Vladimirovich.
Split into simpler sentences: He ruled Tmutarakan under the overal authority of his father Sviatoslav Iaroslavich, Prince of Chernigov. He was twice expelled from his principality by one of his cousins Rostislav Vladimirovich.