From Wikipedia:
Merfyn Frych ('Merfyn the Freckled'; Medieval Latin: Marbinus, Mermin), also known as Merfyn ap Gwriad ('Merfyn son of Gwriad') and Merfyn Camwri ('Merfyn the Oppressor'), was King of Gwynedd from around 825 to 844, the first of its kings known not to have descended from the male line of Cunedda. Little is known of his reign, and his primary notability is as the father of Rhodri the Great and founder of his dynasty, which was sometimes called the Merfynion after him. Merfyn came to the throne in the aftermath of a bloody dynastic struggle between two rivals named Cynan and Hywel – generally identified with the sons of Rhodri Molwynog, despite that putting both men well into their 70s or 80s at the time – at a time when the kingdom had been under pressure from Mercia Gwynedd and neighboring Powys. Both kingdoms were beset by internal dynastic strife, external pressure from Mercia, and bad luck with nature. In 810, there was a bovine plague that killed many cattle (the primary form of wealth at the time) throughout Wales. The next year, the ancient wooden llys at Deganwy was struck by lightning. A destructive war for control of Gwynedd raged between 812 and 816, while in Powys a son of the king was killed by his brother "through treachery". In 818, there was a notable battle at Llanfaes on Anglesey.
Coenwulf of Mercia took advantage of the situation in 817, occupying Rhufoniog and laying waste to the mountains of Snowdonia. Coastal Wales along the Dee Estuary must have remained under Mercia through 821, as Coenwulf is recorded dying peacefully at Basingwerk in that year. In 823, Mercia laid waste to Powys and returned to Gwynedd to burn Deganwy to the ground. Gwynedd and Powys then gained a respite when Mercia's attention turned elsewhere and its fortunes waned. King Beornwulf was killed fighting the East Anglians in 826, his successor Ludeca suffered the same fate the following year, and Mercia was conquered and occupied by Ecgberht of Wessex in 829. Though Mercia managed to throw off Ecgberht's rule in 830, it was thereafter beset by dynastic strife and never regained its former dominance, either in Wales or eastern England.
Merfyn was linked to the earlier dynasty through his mother Ethyllt (or Etthil or Essyllt, Esyllt), the daughter of King Cynan (d. 816), rather than through his father Gwriad ap Elidyr. As his father's origins are obscure, so is the basis of his claim to the throne.
Extremely little is known of Merfyn's father Gwriad. Merfyn claimed descent from Llywarch Hen through him, and the royal pedigree in Jesus College MS. 20 says that Gwriad was the son of Elidyr, who bears the same name as his ancestor, the father of Llywarch Hen, Elidyr lydanwyn. Supporting the veracity of the pedigree is an entry in the Annales Cambriae, which states that Gwriad, the brother of Rhodri the Great, was slain on Anglesey by the Saxons. That is to say, Merfyn named one of his sons after his father Gwriad. Precious little is known of Merfyn's reign. Thornton suggests that Merfyn was probably among the Welsh kings who were defeated by Ecgberht, king of Wessex, in the year 830, but it is unknown how this affected Merfyn's rule.
Merfyn is mentioned as a king of the Britons in a copyist's addition to the Historia Brittonum and in the Bamberg Cryptogram, but as both sources are traced to people working in Merfyn's own court during his reign, it should not be considered more significant than someone's respectful reference to his patron while working in his service.