Magonset, or the land of the Magonsæte was an early Anglo-Saxon kingdom in the region of modern Herefordshire with its capital originally at Kenchester just west of Hereford.
Kings of the Mangonsaete
The names of only three rulers of the kingdom are known;
Kenchester was known to the Romans as Magnis and to the Brythonic rulers that followed as Caer Magnis, which is where the name Magonset comes from. Caer Magnis was probably the centre of an early Brythonic kingdom, but it seems that the focus of the early Celtic church was based at nearby Caerfawydd or Hereford.
Since the name Merewalh name is Old English for 'illustrious Welshman' it seems most likely that Merewalh was a Brythonic king who switched allegiances or simply reached an accomodation with the dominant power of the day.
Certainly sometime in the mid seventh century the area came under the control of Penda of Mercia, and it seems likely that the kingdom of Magonset was always a Mercian dependency.
Around 680 or so Theodore of Tarsus, the Archbishop of Canterbury established a bishopric for the kingdom at Hereford, possibly as a result of a re-organisation agreed at the Synod of Hertford in 673. Merewalh erected a new cathedral at Hereford it is believed (although its location is uncertain) for the new bishop Putta, the former bishop of Rochester, Kent.
Merewalh who died in 685 was succeeded by two of his sons Merchhelm and Mildfrith, although the precise dates of their rule are not known. Nothing further was recorded regarding the kingdom or any subsequent rulers. It is likely that Magonset was simply absorbed into Mercia proper early in the eighth century after the death of Mildfrith, although it remained very a disputed borderland between England and Wales until the time of the Normans.
A Biographical Dictionary of Dark Age Britain by Ann Williams, Alfred P. Smyth and D. P. Kirby (Seaby 1991)
Also an article on Hereford Cathedral at www.britannia.com/history/herefords/churches/herefordcath.html