Ergyng

A kingdom in Wales of the post-Roman and early medieval period, between the 5th and 7th centuries. It's lands lay east of the river Gwy, north of Gwent, and its heartland between the River Mynwy and Gwy, although its borders once extended to the Afon Hafren.
After the withdrawal of the Roman legions from Britain in 410 A.D., new smaller political entities took the place of the centralized structure. The area was originally part of the Kingdom of Glywysing and the Kingdom of Gwent, but become independent for a period under Peibio Clafrog in the late 5th century, during the time of King Arthur. 
Ergyng is the site of the old Roman town Ariconium (Welsh: Ergyng), where it got its name. It was later referred to by the English as Archenfield.