Complex sentence: In 655, Oswiu's forces killed Penda in a decisive victory at the Battle of the Winwæd, establishing Oswiu as one of the most powerful rulers in Britain.
Split into simpler sentences: In 655, Oswiu's forces decisively defeated Penda at the Battle of the Winwæd, where Penda himself was killed. The battle established Oswiu as one of the most powerful rulers in Britain.