592. Æthelfrith, son of Æthelric, succeeds to Bernicia
That Æthelfrith became king of Bernicia in 592 is based on the premises that he was killed at the battle of the River Idle in 616, and that an 8th-century Bernician regnal list gives him a reign-length of 24 years. His father Æthelric is noted in the royal genealogies, and is presumably the Æthelric who ruled Bernicia c.568-72 according to the regnal list. (For more on the regnal list, see entry on c.450-651; for the apparent confusion in the Anglo-Saxon Chronicle, see Miller, pp.46-7.)
Bede reports that Æthelfrith had ravaged the Britons and settled previously-British territory more extensively than any other English king (HE, i.34), and specifically mentions his war against Aedan of Dal Riada which ended in 603, and his battle at Chester in c.613.
Æthelfrith also extended his rule over the neighbouring English kingdom of Deira, so that the Deiran heir, Edwin, was in exile until Æthelfrith's death in 616 (Bede, HE, ii.12). Bede remarks that Ælle (Edwin's father) was still ruling north of the Humber in 597 (Bede's Chronica Maiora, cited by Miller, p.41), so although Æthelfrith's conquest of Deira cannot be dated precisely it was presumably after 597.
The 9th-century Historia Brittonum, at ?63, does claim that Æthelfrith ruled 12 years in Bernicia and 12 years in Deira. At first glance this might suggest a possible date for the Deiran take-over of 604 (12 years after 592), but unfortunately the text goes on to say that Æthelfrith ruled for 24 years in both kingdoms, rather than the 12 years we would expect of a precise account of the joint rule of Bernicia and Deira. This vagueness (and the neatness of the two 12-year halves) makes the passage look more like the guesswork of a later scribe who knew Æthelfrith ruled for 24 years in two kingdoms but had no further information than like a genuine survival from the early 7th century, so the date of Æthelfrith's conquest of Deira is best left unresolved.
M. Miller, "The dates of Deira", Anglo-Saxon England 8 (1979), pp.35-61