Complex sentence: During the 590s Æthelfrith became the most powerful ruler in Northumbria, and following the battle of "Degsastan" in 603 became sufficiently powerful to absorb Deira within his rule.
Split into simpler sentences: During the 590s, Æthelfrith became the most powerful ruler in Northumbria. Following the battle of Degsastan in 603, he became sufficiently powerful to absorb Deira within his rule.