Complex sentence: In 679 he defeated Ecgfrith of Northumbria at the Battle of the Trent: the battle was a major setback for the Northumbrians, and effectively ended their military involvement in English affairs south of the Humber.
Split into simpler sentences: In 679 he defeated Ecgfrith of Northumbria at the Battle of the Trent. The battle was a major setback for the Northumbrians, and effectively ended their military involvement in English affairs south of the Humber.