The Vita Sancti Gundleii (in English the Life of Saint Gwynlliw) is a medieval saint’s life, composed in Cemis, Pembrokeshire, in the 12th century. A copy survies in the British Museum Cotton MS Vespasian A xiv.
According to this account, Gwynlliw was the eldest son of Glywys, King of the Southern Britons. Upon Glywys’ death, his kingdom was divided among his sons, with Gwynlliw getting the kingdom of Glamorgen. Gwynlliw also ruled as chief over his brother kings as a counselor and leader, all of whom respected him. Gwynlliw ruled justly and peacibly. When war came, Gwynlliw was always victorious.
Gwynlliw married Gwladys the daughter of King Brychan Brycheiniog. According to the Vita sancti Gundleii, Gwynlliw was given Gwladys peacibly by her father, making no mention of the account in the Vita Sancti Cadoci according to which Gwynlliw abducted Gwladys from her father and was aided in the following battle by Arthur, Kay, and Bedwyr.
The eldest son of Gwynlliw is Saint Cadog. When Cadog growss up he often urges his parents to cease being to attached to the royal life. Then one night, when Gwynlliw and Gwladys are in bed, an angelic voice announces to them that, to gain salvation, they should give up their power and wealth to go to live instead in a field by a stream. IThe place where they would see a white ox with a black spot between its horns is that place where they should dwell.
So husband and wife leave their palace and set up by the river Ebbw in a hermitage, leaving their kingdom to Cadog.
Gwynlliw and Gwladys perform great austerities, dressing in hair shirts, eating barley mixed with ashes, washing in the coldest river water in the middle of winter, and so forth. Cadog visits them often and now recommends that Gwladys should move elsewhere to avoid any rise of sexual desire. Gwladys moves to a more secluded habitation.
Eventually Gwynlliw falls ill and asks his son Cadog with Bishop Dubric of Llandaff to come to see him. The two saints minister to his soul and finally Gwynlliw passes away on the fourth day before the Calends of April.