A minor Anglo-Saxon sub-kingdom of the Mercian frontier, roughly coextensive with the later Diocese of Hereford, formed in the area of the former Cornovii tribe after Oswiu of Northumbria's conquest of 656; its best-attested ruler was Merewalh, said to be a son of the Mercian king Penda.
Remained a Mercian dependency throughout its existence and was reincorporated into direct Mercian rule by the later 8th century, its name surviving afterward only as a district designation (Westerna).
General context on the formation of Mercia's minor dependent sub-kingdoms, of which Magonsæte is one of the best-documented examples.