Complex sentence: Cadoc was a student of the Irish saint, Tathai, at Gwent, Monmouthshire, who became a hermit, then proselytized over a large area of Wales and Brittany.
Split into simpler sentences: Cadoc was a student of the Irish saint, Tathai, at Gwent, Monmouthshire, who became a hermit. Cadoc then proselytized over a large area of Wales and Brittany.