Complex sentence: Treniota together with Daumantas assassinated Mindaugas and his two sons, Ruklys and Rupeikis, in 1263, and the state lapsed into years of internal fights.
Split into simpler sentences: Treniota together with Daumantas assassinated Mindaugas and his two sons, Ruklys and Rupeikis, in 1263. The state lapsed into years of internal fights.