Æthelfrith, king of Bernicia, gains control of Deira around 604; the circumstances of this are unknown.
That he gained Deira through conquest is suggested by the exile of Edwin, son of the former king Ælla, and Hereric, Edwin's nephew, who are both notable members of the Deiran royal line; the short five-year reign of Aethelric of Deira, who ruled immediately prior to Æthelfrith's acquisition of Deira, may also indicate conquest.
D. P. Kirby on the other hand suggested that Æthelfrith's rule of both kingdoms may have represented "a formalization of an existing relationship" of cooperation between the two.
Kirby also pointed out that Edwin did not necessarily go into exile immediately, and considered it likely that Æthelfrith's hostility towards him "manifested itself only by degrees".
It is also around 604 that Æthelfrith's son Oswald is born.
Oswald's mother is Acha, daughter of Ælla, and thus Edwin's sister.
Although Bede does not explicitly say Æthelfrith married Acha, it is thought that he did so; he may have married her prior to taking power in Deira, in which case the marriage may have facilitated it, or he may have done so afterwards in order to consolidate his position there.
