The Picts defeated the Northumbrians at the Battle of Dun Nechtain, severely weakening the latter's power in northern Great Britain. Battle of Dun Nechtain: The Battle of Dun Nechtain or Battle of Nechtansmere was fought between the Picts, led by King Bridei Mac Bili, and the Northumbrians, led by King Ecgfrith, on 20 May 685. May 20, 685. Wiki link