As the maternal grandson of the last male descended from the First Gwyedd Dynasty (his mother was Esyllt ferch Cynan Tyndaethwy, an only child), Merfyn came to Anglesey from the Isle of Man in 816. He forcefully ousted Hywel ap Caradog from the island cantref and sent him back to his patrimony of Rhos. One would suspect his first order of business was to meet with the heads of other families who had recognized Cynan Tyndaethwy as their king. After securing their support for his kingship claim, as much by sword-rattling as by any entitlement via his mother, he would have likely turned his attention to the kingdom of Meirionydd, still ruled by descendents of Meirion ap Cunedda. Its king was Cynan ap Brochwel ap Ednyfed. We suspect Merfyn Frych was able to bring those lands into his sphere of influence by agreeing to marry a daughter of Cynan. He was then about 29 years old and unmarried. We suggest it was Nest ferch Cynan ap Brochwel of Meirionydd whom he married, not the mythical "Nest ferch Cadell ap Brochwel of Powys". But that Nest was no heiress (Cynan had a son, Cadwaladr) and she brought no land to her husband or their sons.
When Merfyn died, or was killed, in 844, his son Rhodri was perhaps 24/26 years old. It is possible that Merfyn's brother Cadrod acted as interim king until Rhodri attained "full age"... 1
The 13th century History of Gruffydd ap Cynan names "Rodri Mawr son of Mervyn Vrych son of Gwryat…".
King of Gwynedd. The Annales Cambrić record the death in 844 of "Mermin". The Chronicle of the Princes of Wales records the death in 844 of "Mervyn the Freckled". 2