ZEthelfrith eventually acquired possession of Deira, probably in 604 or 605, perhaps on Ella's death, expelling his son Edwin (q.v.).
His position was assured, at least temporarily, in 617, when he decided to espouse the cause of the Northumbrian prince Edwin, then a fugitive at his court, and defeated zEthelfrith of Northumbria on the banks of the Idle, a tributary of the Trent, in Mercian territory.
ZEthelfrith, king of Bernicia, united Deira to his own kingdom, probably about 605, and the union continued under his successor Edwin, son of Ella or Ã†lle, king of Deira.
After the battle of Chester, in which ZEthelfrith defeated the Welsh, Edwin fled to Rcedwald, the powerful king of East Anglia, who after some wavering espoused his cause and defeated and slew IEthelfrith at the river Idle in 617.