He was King of Gwynllwg in South Wales and is the legendary founder and patron saint of the City of Newport living around the 5th century. According to medieval tradition he was a feared warlord and raider who knew King Arthur, but later found religion and became a hermit founding St Woolos Cathedral in Newport. He was the father of one of the most revered Welsh saints, Saint Cadoc the Wise.
The saints' lives portray King Gwynllyw as an active and merciless warrior who attacked and raided nearby kingdoms. The Life of Saint Cadoc describes him as "very partial to thieves, and used to instigate them somewhat often to robberies" but the Life of Saint Gwynllyw insists he was a just and fair ruler. These raids included attacking his northern neighbour Brycheiniog. In one such raid described in Life of Saint Cadoc Gwynllyw accompanied by 300 men abducted Gwladys (Gladys) the beautiful daughter of King Brychan of Brycheiniog, as Brychan had refused to let him marry her. She was one of Brychan's famous twenty-four children. A pitched battle occurred which was only stopped by the intervention of King Arthur and Cai and Bedwyr who supported Gwynllyw and his warband in the battle. This tale of abduction seems similar to elements in the tale Culhwch and Olwen and other Arthurian stories indicating it originated in bardic stories. This is the earliest reference to Arthur in a Saint's life. According to the Life of Saint Gwynllyw this battle never occurred and the marriage was actually accomplished peacefully.
Gwaldys soon had a son, the famous saint Cadoc. To celebrate his son's birth Gwynllyw went another raid stealing cattle from Caerwent. When Saint Tatheus came courageously to demand the return of a cow, the King was so impressed he decided in return to send his son to Tathyw at Caerwent to be educated. Gwynllyw supposedly had other children also saints Cynidr, Bugi and Egwine. Bugi was married to Peren verch of King Lot/Llawdden/Greidal ap Arthwys.
Once grown Cadoc was deeply religious and according to some sources it was his example and preaching that persuaded Gwynllyw to abandon his life of violence and seek forgiveness for his sins. King Gwynllyw then had a dream in which an angel spoke to him and he saw a vision of a white ox with a black spot on its high forehead. Gwynllyw went forth and when he saw the same ox as in his dream he founded a hermitage there on what is now Stow Hill in Newport, South Wales which he built out of wood. Gwynllyw said of the spot: "There is no retreat in the world such as in this space which I am destined now to inhabit. Happy therefore is the place, happier then is he who inhabits it." Gwynllyw's decision to abandon his kingship and retire to a religious life seems to have been a common theme amongst Welsh saints and even his violent past was not unusual, being shared by Saint Illtyd amongst others.
Gwladys accompanied Gwynllyw into a hermits life and for a while they lived together on Stow Hill, fasting, eating a vegetarian diet, and bathing in the cold waters of the Usk to prove their piety. A miraculous fountain started on the hill when Gwynllyw prayed for water. Later they moved further apart, Saint Gwladys founding her own hermitage at Pencarn.
When Gwynllyw was dying he was attended both by his son Cadoc and Saint Dubricius, who administered the last sacrament to him. The traditional date for his death is 29 March is the day dedicated to him. The year of his death is uncertain, suggestions include 500 and 523. Following his death his hermit cell became an important shrine and a church was built there. This is now St Woolos Cathedral, the seat of the Bishop of Monmouth. In the 9th century Gwynllyw's church was rebuilt in stone. This indicates his importance and the wealth of his shrine as stone buildings were unusual in Wales at this point. Part of this building is now incorporated into St. Woolos Cathedral as the Galilee chapel.
Stories of Gwynllyw's dark past later included tales of piracy and claims that his ships, based in the Uskside parish of Pillgwenlly that bears his name, caused terror across the Bristol Channel. One tradition asserts that this background meant Gwynllyw was the patron saint of choice for Welsh pirates and smugglers including Sir Henry Morgan. Certainly the many sailors based in Newport would have known of him. Another local story claims that Gwynllyw forcibly baptised the population of Gwynllwg by the sword.
In 1949 St Woolos Church became a full cathedral and, besides churches, the saint is remembered today through St Woolos Hospital and in 1988 a Welsh language school Ysgol Gyfun Gwynllyw was set up in Pontypool. The vision of an ox inspired the sculpture by Sebastien Boyesen called The Vision of Saint Gwynllyw or The Bell Carrier, finished in 1996 and found in central Newport today.