Hywell was the youngest son of Cadell, himself the son of Rhodri the Great.
909 — King Cadell son of Rhodri dies. 915 — King Anarawd dies. 919 — King Clydog (his brother and co-ruler of Seisyllwg or Ceredigion)was killed. Slain by another brother, Meurig 928 — Hywel journeyed to Rome.
Hywel Dda (English: Hywel the Good) or Hywel ap Cadell (c.880 – 950) was a King of Deheubarth who eventually came to rule most of Wales. He became the sole king of Seisyllwg in 920 and shortly thereafter established Deheubarth, and proceeded to gain control over the entire country from Prestatyn to Pembroke. As a descendant of Rhodri Mawr through his father Cadell, Hywel was a member of the Dinefwr branch of the dynasty. He was recorded as King of the Britons in the Annales Cambriae and the Annals of Ulster.
Hywel was a well thought of King of Deheubarth in south-west Wales who came to rule Wales from Preststyn to Pembroke. A descendent Of Rhodri Mawrthrough thru his father Cadell, Hywel was a member of the Dinefwr branch of the dynasty.
He is remembered as one of the most responsible Welsh rulers of all time. His name is particularly linked with the development of the Welsh laws, generally known as the 'Laws of Hywell Dda'.
A well educated man by our standard's having a good knowledge of Welsh, Latin, and English.
Alliance with Athelstan
Hywell achieved an understanding with Athelstan of England where they both ruled a part of Wales jointly. Hywel minted his own coinage in the English City of Chester. He was the first Welsh ruler to produce coinage for at least a thousand years.
928 Pilgrimage to Rome
His study of legal systems and his pilgrimage to Rome in 928 combined to enable him to formulate advance ideas about law.
The Hywel "Law" book was written partially in Latin, about laws at court, law of country, and law of justices.
948. And Howel the Good, son of king Cadell, chief and glory of all the Britons, died. And Cadwgan, son of Owain, was killed by the Saxons. And then the action of Carno took place between the sons of Owain, son of Howel, and the sons of Idwal.
950 - Hywel king of the Britons died.. Boyer, following Bartrum, also gives a death year of 950. 
Cawley reports that Hywel & his wife had four children. 
Boyer lists nine children, identifying which are listed by Bartrum and which by Bridgeman. The list below follows the order in Boyer. Dates currently listed in Wikitree but without source are identified. 
Listed by Bartrum
Owain ap Hywel born Dynevor 913 Owain married Angharad ferch Llywelyn ap Merfyn ap Rhodri MawrListed as a child by Bartrum  Owain ap Hywel (-[987/88]). [The Gwentian Chronicle records that "his son Owain took the rule of Ceredigion" after the death of "Hywel the Good, son of Cadell king of all Wales" in 948.] King of Gwynedd, jointly with his brothers. One of four children of Hywel and his wife reported by Cawley. 
Dyfnwal ap Hywel no date or place of birth  Dyfnwal, died 953, had son Owain, died 990. Listed as a child by Bartrum  Dyvnwal. The Chronicle of the Princes of Wales records that "Dyvenwal and Rhodri sons of Howel" died in 951. One of four children of Hywel and his wife reported by Cawley. 
Rhodri ap Hywel no date or place of birth Rhodri, died 953, had son idwal, died 962. Listed as a child by Bartrum  Rhodri. King of Gwynedd, jointly with his brothers. The Annales Cambriæ record the death in 954 of "Rotri filius Higuel". The Chronicle of the Princes of Wales records that "Dyvenwal and Rhodri sons of Howel" died in 951. One of four children of Hywel and his wife reported by Cawley. 
Listed by Bridgeman but not by Bartrum
Edwyn (or Gwyn) DNB 10:106, died c. 952 or 954 (NLWJ, 13:134; his mother may have been an English mistress. Listed as a child by Bridgeman but not by Bartrum.  It is notable that he gave one of his sons an Anglo-Saxon name - Edwin Edwin. King of Gwynedd, jointly with his brothers. The Chronicle of the Princes of Wales records that "Edwin son of Howel the Good" died in 952. One of four children of Hywel and his wife reported by Cawley. 
Cynan. Listed as a child by Bridgeman but not by Bartrum. 
Maredith (Maredudd). Listed as a child by Bridgeman but not by Bartrum. 
↑ Darrell Wolcott, The Clan of Tudor Trevor. http://www.ancientwalesstudies.org/id55.html Accessed August 26, 2015. Welsh genealogies are long on pedigrees and short on dates. As a result, the dates sometimes attached to individuals conflict with dates attached to their parents or children. Darrell Wolcott, President, Center for the Study of Ancient Wales, has constructed estimated birth years for those coming before and following Tudor Trevor which seek to resolve the conflicts. Born in Jefferson, Texas in 1935 Darrell Wolcott grew up in Ohio. After a career in banking he took early retirement in 1996, and returning to his place of birth he created and endowed the Center for the Study of Ancient Wales (a non-profit foundation with a library of some 3200 volumes) located in Jefferson, Texas. The foundation has constructed a small castle to house its library and offices and acquired copies of thousands of source and secondary works related to the history of Wales, many of which are otherwise unavailable outside Britain. At present he is its sole researcher.