Complex sentence: In 679, Aelfwine, the brother of King Ecgfrith of Northumbria, was killed in battle against the Mercians, and Theodore intervened to make peace between the two kingdoms by persuading King Æthelred of Mercia to pay "weregild" compensation for Aelfwine's death.
Split into simpler sentences: In 679, Aelfwine, the brother of King Ecgfrith of Northumbria, was killed in battle against the Mercians. Theodore's intervention prevented the escalation of the war and resulted in peace between the two kingdoms, with King Æthelred of Mercia paying "weregild" compensation for Aelfwine's death.