Its first unified king was Athelfrith of Bernicia, who conquered Deira around 604CE. He was killed in battle around 616 by Raedwald of East Anglia, who installed Edwin, son of a former Deiran king, as Northumbrian king.
Edwin accepted Christianity in 627CE, and conquered the Isle of Man and the North Welsh kingdom of Gwynedd before being defeated by an alliance of Gwynedd's exiled king, Cadwallon, and Penda, king of Mercia. The two then proceeded through Northumbria, defeating two putative successors to Edwin, Osric and Eanfrith. Cadwallon sought to set himself up as the king, but in 634-5 Eanfrith's brother Oswald rose up and killed him at Heavenfield, near Hexham. Eight years later Penda, in turn, defeated and killed Oswald, but fell, in 655, in battle, to one Oswiu (who may have been Oswald's relative).
After taking power in Northumbria (and marrying Edwin's daughter Eanflaed), Oswiu went on to take the throne in Mercia (then England's leading kingdom) but was overthrown there by Penda's son Wulfhere, and limited to his North-Eastern realm. Here he was most notable for accepting Roman ecclesiastical dating, over the Celtic alternative, at the Synod of Whitby. He died in 670CE and was succeeded by Ecgfrith his son.