In 604 Athelfrith attacked Deira to his south. King Athelric of Deira was killed. Athelfrith also tried to kill Edwin of Deira, Athelric's brother, but Edwin escaped. He took Edwin's sister, Achaas, as his second wife. In 605 Athelfrith united the two kingdoms, Bernicia and Deira, into a single kingdom called Northumbria. He must have spent the next few years setting up ways to govern such a large territory. He almost certainly set up fortifications as well. By 613 Athelfrith began trying to capture Edwin who had since been in exile in Gwnedd and Powys. He moved his army across the Pennines to Chester. When several monks were sent by the Britons to negotiate with Athelfrith they were killed. Between 613 and 616 Athelfrith defeated the Britons in battle at Chester. Edwin escaped again this time to Mercia, then to East Anglia. Athelfrith wanted the king, Raedwald of East Anglia, to murder Edwin, but he would not. Concerned that Athelfrith might attack East Anglia, Raedwald took his army and attacked Athelfrith. The two armies met at the River Idle near Doncaster. Not seeing the East Angles as a major threat, Athelfrith brought a smaller army. Athelfrith was killed after he was separated from his bodyguards. Edwin returned to Northumbria to become the king. At Athelfrith's death his family fled north to find safety. Eanfrith, the eldest son, found safety with the Picts. He married the sister of Gartnait, King of the Picts. His Oswald and Oswiu found safety with the Scots in Argyll.