Bede's Ecclesiastical History of England

In the ninth year of the reign of King Egfrid, a great battle674 was fought between him and Ethelred, king of the Mercians, near the river Trent, and Aelfwine,675 brother to King Egfrid, was slain, a youth about eighteen years of age, and much beloved by both provinces; for King Ethelred had married his sister Osthryth.676 There was now reason to expect a more bloody war, and more lasting enmity between those kings and their fierce nations; but Theodore, the bishop, beloved of God, relying on the Divine aid, by his wholesome admonitions wholly extinguished the dangerous fire that was breaking out; so that the kings and their people on both sides were appeased, and no man was put to death, but only the due mulct677 paid to the king who was the avenger for the death of his brother; and this peace continued long after between those kings and between their kingdoms.