Cynog is invariably described as the eldest son of Brychan Brycheniog, the great father of a saintly family. According to legendary account, King Tewdrig of Garthmadrin (the old name for Brycheniog) had a only daughter named Marchell, who went to Ireland where she married Anlach son of Coronac.
There is preserved a poem “Cywydd Cynog Sant” by the bard and historian Hywel ab Dafydd ab Iefan ab Rhys, who lived in the second half of the fifteenth century and was evidently connected with Brecknockshire. In this poem he addresses the Saint as Cynog of Breconia who had been left by Brychan supreme governor of that country. He had in earlier life he says, refused dominion and crown in Ireland “a prosperous brilliant crown” but for the love of God, he chose the hermit life instead. When he came over to this island he encountered in Caer Wedros (South Cardiganshire) a fiendish giant addicted to cannibalism that infested it. To spare a victim he allowed the giant to cut a large slice of his own thigh and over the place grew “a sheep’s white wool”. The fiend relishing it came again for a slice but Cynon slew him with his “torque from heaven” fashioned in of red-yellow metal without the operation of a smith’s hand. A smith of “Evena” one broke in three but it was miraculously pieced together again.
His martyrdom is related to his later years for his desires for fixed solitude for meditation he retired into is his own county and adjoined himself to the society of certain religious men that led a hermitical life under the government of a superior in little cells upon a hill called the Van about 4 miles from Brecknock and about 2 miles from Carevong the place of his birth which is now destroyed and called the Gare where he built him a hermitage under a steep Rock near the top of the mountain. These men lived by the labour of their hands and had no water but what they fetched from the river at the foot of the mountain which was very troublesome to carry up the hill.
This made them moan and disrespect strangely while this holy man underwent the labour with all cheerfulness and reprimanded them for their murmurings and laziness against a labour in which on its self was holy and worked as exceeding reward. This raised their whole bitterness again Cynog, while god considering his age and good will gave him water upon the top of the rock over is little cell where he mounted every morning for meditation and prayer where no man else could have any this fuelled their rage thinking themselves mocked by him that they resolved to murder him, where upon two of them mounting the rock one Sunday morning found him as his prayers and saw the crystal spring at his feet they furiously ran at him and cut of his head with a sword which dropt into the well where the water immediately gave way to his head and dried up, nor would god almighty suffer these wicked monks to triumph over the sacred head which had humbled itself so much for its sake, for his dead body assisted by his holy spirit immediately took up his head in his hands and carried it down the hill from thence he walked on to a rising ground about a fields breadth beyond the church and laid it down under a bush of brambles.