Writing in the 1130s, Geoffrey of Monmouth mentions the Battle of Winwaed in his History of the Kings of Britain (Historia Regum Britanniae) [xii.13]:
Cadwallo was influenced by this speech [by one of his advisers] and by what the others said. He gave Peanda [Penda] permission to fight with Oswi. Peanda then collected together a huge army and crossed the Humber. He launched a bitter attack against King Oswi and devastated the various districts of his country. Oswi, who was reduced in the end by sheer necessity, promised Peanda innumerable royal decorations and more presents than anyone could ever imagine, if only he would stop destroying his country, put an end to the invasion, and go back home. Peanda refused pointblank to yield to Oswi's entreaties. King Oswi then put his trust in God's help and joined battle with Peanda on the bank of the River Wunued, although he himself had a smaller army. Peanda and thirty of his leaders were killed and Oswi won a victory.
Geoffrey of Monmouth can be very unreliable, as he mixes myths with existing historical accounts. His account of the battle by Wunued River, bears an uncanny resemblance with that of Bede. Geoffrey knew of (and elsewhere referenced) Bede's Ecclesiastical History, so there is the possibility that he copied Bede's account of the battle.
Geoffrey's account is from the other side of the battle to that of Bede's. He is interested in Cadwallo (King of the Britains), rather than the growth of Christianity amongst the Anglo-Saxons. Geoffrey's Cadwallader was the son of Cadwallo. Geoffrey's Cadwallo had previously invaded Mercia and captured King Peanda. In return for an attack on the Saxons, Peanda became a puppet king.
This is where Geoffrey of Monmouth and Bede differ. Bede's Cadwalla is a tyrant ("infamous British King") who is killed by Oswald at an earlier date.
Aaron Thompson suggests in his translation that Geoffrey's River Wunued was actually the River Aire.
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