Cadell ap Rhodri was the son of Rhodri Mawr ("the Great")

Rhodri the Great

Rhodri the Great was King of Gwynedd from 844 until his death. He was the first Welsh ruler to be called 'Great', and the first to rule most of present-day Wales...

. He inherited the kingdom of Seisyllwg from his mother Angharad of Seisyllwg in 878, and passed it to his son, Hywel Dda ("the Good")

Hywel Dda

Hywel Dda , was the well-thought-of king of Deheubarth in south-west Wales, who eventually came to rule Wales 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 and is also named Hywel ap Cadell...

, on his death in 909. Cadell and Hywel had previously conquered Dyfed

Dyfed

Dyfed is a preserved county of Wales. It was created on 1 April 1974 under the terms of the Local Government Act 1972, and covered approximately the same geographic extent as the ancient Principality of Deheubarth, although excluding the Gower Peninsula and the area west of the River Tawe...

 in 904/905, establishing Hywel as the king in that region. After his father's death, Hywel merged the two kingdoms to establish Deheubarth.