Originally it comprised two independent kingdoms, Bernicia and Deira.
The first king of Deira of whom we know was Ella, or Aelle, who, according to Bede, was still reigning when Augustine arrived in 597.
At his death in 633 the kingdom was again divided, Deira falling to his nephew Osric, while Bernicia was occupied by Eanfrith son of ZEthelfrith.
After Oswald's defeat and death at the hands of Penda in 642 Bernicia fell to his brother Oswio, while Oswine son of Osric became king in Deira, though probably subject to Oswio.
Oswine's death was compassed by Oswio in 651, and the throne of Deira was then obtained by 1-Ethelwald son of Oswald.