The Annales Cambrić record that "filius Teudur Resus" started to rule in 1077. The Gwentian Chronicle records that "Rhys son of Tewdwr came from Llydaw and put in a claim to the principality of South Wales as lawful heir" in 1077.
He succeeded in 1078 as King of Deheubarth. The Chronicle of the Princes of Wales records that "Rhys son of Tewdwr began to reign" in 1077. Gerald of Wales´s Descriptio Kambrić names “descendientes...a Theodoro...Resus filius Theodori, Griphinus filius Resi, et Resus filius Griphini qui hodie praest” as successive rulers in South Wales. The Annales Cambrić record that "Resus filius Teudur" was expelled from his kingdom by "filiis Bledint, scilicet Madauc, Cadugan et Ririt" in 1087. The Chronicle of the Princes of Wales records that "Rhys son of Tewdwr was expelled from his territory by the sons of Bleddyn, sons of Cynvyn, to wit Madog, and Cadwgan, and Rhirid, and he himself retreated into Ireland, and immediately afterwards he collected a fleet of the Gwyddelians and returned again, and then the battle of Llych Crei took place, and the sons of Bleddyn were slain" in 1087, adding that "Rhys son of Tewdwr gave an immense sum of money to the mariners, the Scots and Gwyddelians who had come to assist him".
Florence of Worcester records that "Res Walanorum rex" was killed in battle during Easter week "iuxta castellum Brecheniean" in , after which "kings ceased to reign in Wales". The Chronicle of the Princes of Wales records that "Rhys son of Tewdwr king of South Wales was killed by the French, who inhabited Brecheiniog, and then fell the kingdom to the Britons" in 1091. 1
RHYS ap TEWDWR (d. 1093 ), king of Deheubarth ; grandson of Cadell ab Einion ab Owain ap Hywel Dda . In 1075 he took possession of Deheubarth on the death of his second-cousin, Rhys ab Owain ab Edwin . In 1081 he was dislodged by Caradog ap Gruffydd , but later in the year, with the help of Gruffudd ap Cynan , he was firmly reinstated after the historic battle of Mynydd Carn . In the same year William the Conqueror made a demonstration of power in South Wales , traversing the land as far as S. Davids ; it is reasonably certain that during the visit the two kings came to an agreement as to their future good relations, which lasted to the end of William 's reign. A few years later it is recorded that Rhys is paying the king £40 a year for Deheubarth , thereby becoming a vassal of the Norman Crown and establishing a precedent with lasting consequences on Anglo-Welsh relations.
Henceforth, with the exception of the closing tragedy of his career, Rhys had only to contend with the jealousies of his fellow princes. In 1088 he was attacked by the young rulers of Powys and was obliged to seek refuge in Ireland , but he soon returned and, with Danish help, decisively defeated his opponents (see Madog , Rhiryd , and Cadwgan ap Bleddyn ). Again in 1091 he was opposed by a group of his own vassals in Dyfed , who sought to restore the kingship to the senior line of Hywel Dda in the person of Gruffydd ap Maredudd ab Owain . At Llandudoch ( S. Dogmaels ) on the Teifi the rebels were defeated and Gruffydd killed. Meanwhile the Norman conquest of the south had gathered a new momentum after William 's death in 1087 , and among the territories then being over-run was the old kingdom of Brycheiniog . It was while resisting the Norman advance in this all-important approach to his own dominions that Rhys was killed in uncertain circumstances near Aberhonddu ( Brecon ).
He was virtually the last of the ancient kings of Deheubarth , and it was in a different political setting that the power of the dynasty was eventually revived by his grandson — Rhys ap Gruffydd . He m. Gwladus , daughter of Rhiwallon ap Cynfyn . He was survived by two sons, Gruffydd ap Rhys and Hywel , and by a daughter, Nest . 2