Complex sentence: Threatened by Harold's fleet, Tostig moved north and raided in East Anglia and Lincolnshire, but he was driven back to his ships by the brothers Edwin, Earl of Mercia, and Morcar, Earl of Northumbria.
Split into simpler sentences: Threatened by Harold's fleet, Tostig moved north and raided in East Anglia and Lincolnshire. He was driven back to his ships by the brothers Edwin, Earl of Mercia, and Morcar, Earl of Northumbria.