Dumnonia

A sub-Roman kingdom and region of the southwestern peninsula of Britain that flourished in the early dark ages, especially during the time of King Arthur. It contained the lands of Cornwall (Kernow), and further eastward into the region the Welsh called Dyfnaint, the Celtic lands of Dan-munith, and Durnovaria. During this time Dumnonia was ruled by King Erbin and his son Geraint, descendants in a line of Cornish and Roman-British rulers.   
Dumnonia was named by the Romans after the Dumnonii, a Celtic tribe living in the south-west at the time of their invasion of the island.