However, it also spread into modern Monmouthshire and east of the Wye, where sits the old Roman town of Ariconium (Welsh: Ergyng) at Weston under Penyard from which its name may derive; it may have been the first capital.

[2] Peibio was the grandfather of Saint Dubricius (or Dyfrig), the first Bishop of Ergyng and an important figure in the establishment of Christianity in South Wales.

He founded large teaching monasteries at Llanfrother near Hoarwithy and at Moccas, and a bishopric seems to have been based at St Constantine's Church at Goodrich.

In the middle of the 7th century,[dubious – discuss] Onbraust of Ergyng married Meurig of Gwent, and their son Athrwys became king of both kingdoms.

Although its Welsh-speaking inhabitants retained special rights, the area was unequivocally incorporated into the English county of Hereford in the Laws in Wales Acts of 1535 and 1542.