Deheubarth was a southern Welsh kingdom, arising from the former kingdoms of Dyfed and Seisyllwg in 920 AD, under the rule of Hywel Dda.   At various times, it fell under the auspices of Gwynedd, namely, during the rule of Gruffydd ap Llywelyn in 1055 AD.  The Norman conquest, as for the Saxons to the east, was not a happy event, however, and Deheubarth fell to them before 1100 AD.  These Normans conquered the southern regions of Wales more fully than they ever did the north, including Deheubarth (until 1282, at which point Edward I conquered all of Wales).

The Normans accepted a client rule in certain instances and granted Cantref Mawr to Gruffydd ap Rhys in 1116. In time, he passed its rule onto his son, Anarawd.  With the help of Owain Gwynedd, Anarawd and Gruffydd successfully revolted against their Norman masters in 1136.  Gruffydd was killed in battle in 1137 and Anarawd went on to rule Deheubarth until he was murdered by Owain Gwynedd’s brother, Cadwaladr, in 1143.

“The Welsh lawbooks of the medieval period, the earliest of which is a text of the 13th century, accorded to Dinefwr a special status as the principal court of the kingdom of Deheubarth. Indeed, the lawbooks which emanate from the kingdom of Deheubarth accord Dinefwr parity with Aberffraw, the chief court of the kingdom of Gwynedd. The phraseology of the lawyers’ statements may give Dinefwr an aura of antiquity, but written sources do not suggest that the castle has any history earlier than the 12th century. The earliest reference to the castle at Dinefwr in historical sources belongs to the period of Rhys ap Gruffydd, the Lord Rhys. One of the greatest Welsh leaders of the 12th century, Rhys ap Gruffydd was able to withstand the power of the Anglo-Norman lords of the March, supported on occasion by the intervention of King Henry II (1154-89) of England, and recreate the kingdom. He was then able to take advantage of the king’s more conciliatory policy in the period after 1171 to maintain stable authority for many years. Deheubarth flourished over a period of relative peace and general harmony, with Welsh culture and religious life, as well as legal and administrative affairs, all benefiting from Rhys’s patronage and self-assured governance.”  http://www.castlewales.com/dinefwr.html

A younger brother, Rhys ap Gruffydd, ruled from 1155 to 1197 and submitted to King Henry in 1158 for the right to it.  He was known from then on as “Lord Rhys”.  After his death, “the princes of Deheubarth were effectively minor lords subject to Gwynedd and ruling small commotes and cantrefs in Ystrad Tywi and Ceredigion with no real authority. Rhys ap Maredudd was the last to make a stand in the South and was briefly proclaimed lord of Ystrad Tywi.”  http://www.castlewales.com/debarth.html

This stand by Rhys ap Maredudd came in 1287, after the death of Llywelyn ap Gruffydd.  Rhys had supported King Edward, despite overtures from Llywelyn.  “But the actions of the royal officials of the shire irked him, and moreover he had a feud with the Giffards of Iscennen ( Llandovery ). His grievances, however, as T. F. Tout puts it, were ‘those of a Marcher rather than those of a Welshman .’  http://yba.llgc.org.uk/en/s1-RHYS-APM-1291.html  He was eventually caught and executed in 1291.

These are the rulers from the 10th century on:

Kings of Deheubarth
Hywel Dda the Good   909-950
Rhodri ap Hywel 950-953 (joint)
Edwin ap Hywel 950-954 (joint)
Owain ap Hywel 950-987
Maredudd ab Owain 987-999
Cynan ap Hywel 999-1005
Edwin ab Einion 1005-1018
Cadell ab Einion 005-1018
Llywelyn ap Seisyll 1018-1023
Rhydderch ab Iestyn 1023-1033
Hywel ab Edwin 1033-1044
Gruffydd ap Llywelyn 1044-1047
Gruffydd ab Rhydderch 1047-1055
Gruffydd ap Llywelyn (again) 1055-1063
Maredudd ab Owain 1063-1072
Rhys ab Owain 1072-1078
Rhys ap Tewdwr 1078-1093
Gruffydd ap Rhys 1135-1137
Anarawd ap Gruffydd 1137-1143
Cadell ap Gruffydd 1143-1153
Maredudd ap Gruffydd 1153-1155
Rhys The Lord Rhys ap Gruffydd 1155-1197
Gruffydd ap Rhys 1197-1201