Complex sentence: In 1263, Treniota (and Daumantas?) assassinated Mindaugas and two of his sons, and the Grand Duchy of Lithuania relapsed into paganism for another one hundred and twenty years.
Split into simpler sentences: In 1263, Treniota assassinated Mindaugas and two of his sons. As a result the Grand Duchy of Lithuania relapsed into paganism for another one hundred and twenty years.