1911 Encyclopædia Britannica/Æthelfrith
ÆTHELFRITH, king of Northumbria, is said to have come to the throne in A.D. 593, being the son of Æthelric (probably reigned 568–572). He married Acha, daughter of Ella (Ælle), king of Deira, whom he succeeded probably in 605, expelling his son Edwin. In 603 he repelled the attack of Aidan, king of the Dalriad Scots, at Daegsastan, defeating him with great loss. The appearance of Hering, son of Hussa, Æthelfrith’s predecessor, On the side of the invaders seems to indicate family quarrels in the royal house of Bernicia. Later in his reign, probably in 614, he defeated the Welsh in a great battle at Chester and massacred the monks of Bangor who were assembled to aid them by their prayers. This war may have been due partly to Æthelfrith’s persecution of Edwin, but it had a strategic importance in the separation of the North Welsh from the Strathclyde Britons. In 617 Æthelfrith was defeated and slain at the river Idle by Raedwald of East Anglia, whom Edwin had persuaded to take up his cause.
See Bede, Chronica Mojora, sec. 531; Hist. Ecc. (Plummer) i. 34, ii. 2; Saxon Chronicle, s.a. 593, 603, 605, 616; Hist. Brittonum, sec. sec. 57, 63 Annales Cambriae, s.a. 613.