In the year 1086 Morgannwg was an independent Welsh kingdom, by the year 1093 it had been conquered by one Robert Fitz Hamon and organised as a Norman Marcher Lordship.
Myth
There is an entirely spurious account of the conquest invented by Elizabethan antiquarians such as Powel in his The Historie of Cambria. This traditional and romantic account begins with a genuine historical event, that of the battle of Llandudoch where Gruffudd ap Maredudd ab Owain failed in his attempt to oust Rhys ap Tewdwr from Deheubarth. Gruffudd who died in the attempt was supported by two sons of one Cedifor ap Gollwyn, a lord of Deheubarth, named Einion and Maredudd.
According to this fictional account Einion survived the battle, and fled to the safety of Iestyn ap Gwrgan, king of Morgannwg who had his own argument with Rhys ap Tewdwr. There Einion agreed to help secure the services of a Norman army to assist Iestyn in his fight against Rhys in return for the promise of Iestyn's daughter in marriage. Einion duly secured the services of one Robert Fitz Hamon, and twelve other Norman knights. Together this allied force raided Deheubarth and then at the battle at Bryn-y-Beddau defeated and killed Rhys together with his two sons, Goronwy and Cynan.
Iestyn then paid off the Normans but reneged on his bargain with Einion. So Einion contacted the Normans once again and persuaded Robert Fitz Hamon to join with him in attacking Iestyn. At the battle of Mynydd Bychan near Cardiff, Iestyn was defeated and killed. Thus Robert Fitz Hamon gained Morgannwg and Einion was granted the lordship of Senghenydd as his reward and became thereafter known as 'Einion Fradwr' or 'Einion the Traitor'.
(We know the tale is fictional, if only because we know that Rhys ap Tewdwr really died in Brycheiniog at the hands of Bernard of Neufmarche.)
Reality
There is unfortunately, no accurate and detailed historical account to relate in place of this fiction.
The evidence of the Domesday Book of 1086 confirms that the Normans did not hold any territory within Morgannwg at that time and we can be reasonably certain that Robert Fitz Hamon did not launch his offensive until after he was in possession of his lands at Gloucester which he received in 1088. But other than that, the dating of the conquest is unsecure. Some historians have argued that the conquest of Morgannwg did not begin until after the death of Rhys ap Tewdwr 1093; others argue that there it began shortly after 1088 and was roughly contemporaneous with the conquest of Brycheiniog.
It is generally accepted that Morgannwg was gained through a sea-borne invasion from across the Bristol Channel, led by Robert Fitz Hamon from his power base in Gloucester , but the precise details of how and when he defeated the incumbent ruler Iestyn ap Gwrgan is not known.
What is known, is that by the year 1093,
Robert Fitz Hamon had taken personal control of the lordships of
Cardiff,
Cowbridge, and
Kenfig, which all lay alongside the
Via Juliana, the old
Roman road that ran between
Caerleon and
Carmarthen and built castles at both
Cardiff and
Kenfig. He also controlled the lordships of
Meisgyn (in the
Taff valley) and
Glyn Rhondda, and the lordship that became known as 'Tir yr Iarl' (the Earl's Land), lying between the
Afan and
Ogmore rivers. Other lordships in the lowland
Vale of Glamorgan were handed out to sundry
Norman knights, but the upland areas were allowed to remain under the direct control of
Welsh rulers.
The teritory of Senghenydd, was granted to one Einion ap Cedifor, the very same 'Einion Fradwr' of the fictional account related above and a further two lordships were granted to two sons of Iestyn ap Gwrgan. Caradog ap Iestyn, the eldest son, retained the Welsh commote of Afan which was transformed into the Norman lordship of Avon. Caradog built himself a castle on the banks of the river and established a borough at Aberafan, enthusiastically joined the emerging Cambro-Norman aristocracy and adopted the family name of Avene.
Which is the way the Normans generally did things in Wales; when they had to, they fought; but they married the daughters of the Welsh kings they supplanted, and sought to co-opt the sons into the new aristocracy they planted.
SOURCES
The Normans in South Wales 1070-1171 by Lynn H. Nelson (University of Texas Press, 1966)
A History of Wales by John Davies (Allen Lane 1993)
The Normans by David C Douglas (Folio, 2002)
The Welsh Kings by Kari Mundi (Tempus 2000)