Eagles Have Flown
King of Dumnonia
The Celtic tribe of the Dumnonii ruled a large kingdom that covered the whole of the West Country from Somerset onwards and probably began to emerge as a distinct region by the beginning of the fourth century. It had become a fully independent kingdom by the fifth century, probably by 410.
In this area, scarcely touched by Roman occupation, the Dumnonian leaders would have exercised a far higher level of self-rule than many areas of Britain. They controlled a wide swathe of territory that also comprised the former territory of the Durotriges and Cornovii, in the modern counties of Dorset and Cornwall respectively.
As well as producing one of the earliest independent kingdoms, Dumnonia was one of the most stable until the West Saxon territorial gains of the late sixth century, neither changing its name or fragmenting in the way of most Celtic kingdoms. It produced a few sub-kingdoms, Glastenning, Cornubia and Lyonesse, which were ruled, in the same manner as Gwynedd, under Dumnonia’s overall control, and were mostly later drawn back under direct rule. Glastenning was the only one not regained in whole, but this was due to Saxon conquests in the region.
In the remote south-west, the English Cornwall derives directly from its Romano-British label. Its Late British name, Cornouia, which was Latinised as Cornubia, has survived as Welsh Cerniw and Cornish Kernow (not to be confused with the Cernyw of Glywyssing). Cornubia seems to have constituted a pagus, a Roman subdivision, within the civitas of the Dumnonii, and later, as stated, a sub-kingdom for a time, before merging with Dumnonia as the latter was reduced by Saxon advances.
from Kessler, P. L. (2007, September 8). Post-Roman Britain: Early Independent Britain AD 400-425. Retrieved from http://www.historyfiles.co.uk/FeaturesBritain/BritishMapAD400.htm