Complex sentence: King Cynegils of Wessex gave Dorchester to Birinus to make it the seat of a new Diocese of Dorchester under a Bishop of Dorchester, which was extremely large, and covered most of Wessex and Mercia.
Split into simpler sentences: King Cynegils of Wessex gave Dorchester to Birinus to make it the seat of a new Diocese of Dorchester. The Diocese of Dorchester was extremely large, and covered most of Wessex and Mercia.