Complex sentence: The Picts, possibly with Scottish and Strathclyde Briton help, defeat the Saxon guard, killing Ecgfrith, who had reigned 15 years, routing his army and forcing the Anglo-Saxons to withdraw south of the River Forth.
Split into simpler sentences: The Picts, possibly with Scottish and Strathclyde Briton help, defeat the Saxon guard. Ecgfrith is killed after a 15-year reign, routing his army and forcing the Anglo-Saxons to withdraw south of the River Forth.