The 'Vita Sancti Cadoci (‘Life of Saint Cadoc’) is a saint’s life in Latin written about the year 1100 by Lifris (or Leofric), son of Herwald, Bishop of Llandaff.
According to this work, Cadoc was son of King Gwynllyw and his mother was Gwladys, one of the many daughters of King Brychan Brycheiniog. When King Brychan refuses Gwynllyw’s attempt to take Gwladys for his wife, Gwynllyw, along with three hundred of his men, abducts Gwladys from before the fortress of Talgarth. When Gwynlliw saw King Brychan and his forces riding after them, Gwynllyw mounts Gwladys with himself on his horse and lead his men in a retreat from King Brychan and his men. Gwynlliw intends to ride to his own army and then turn and fight. About two hundred of Gwynllyw’s men are slain. Gwynllyw finally reaches the border of his own country at the hill Boch Rhiw Carn (‘Cheek of the Stony Way’).
It chanced that Arthur with Kay and Bedwyr are now at the top of that hill, playing dice. Seeing the Gwynllyw’s battling troops and seeing Gwladys’ beauty, Arthur thinks to take the woman for himself. Kay and Bedwyr remind Arthur they have been accustomed to aid the needy and distressed and suggest that they should go with all speed to put and end to the fighting. Arthur tells them that if they prefer to aid the rider rather than to take the damsel, they should first determine who is the owner of the country in which they are.
When Kay and Bedwyr ask him, Gwynllyw swears that he is owner of the land. Accordingly Arthur, Kay, and Bedwyr join his side in the battle and put King Brychan and his men to flight. So Gwynllyw is made safe and is able to take Gwladys to his residence at Allt Wynllyw (‘Gwynllyw’s Hill’).
Gwynllyw marries Gwladys, and their eldest son is Cadog (Cadoc).
Cadog is a very powerful saint both as a child and as an adult, most powerful in his cursing of those who offend.
Cadog has to deal with Arthur when Arthur discovers that Cadog for seven years has been sheltering Ligessauc Long-hand, the son of Eliman, who had fled from Arthur after slaying three of Arthur’s men. Arthur learns where Legessauc is hiding and comes to the River Usk with a great force. But fearing the spiritual power of Cadog, Arthur submits the matter to arbitration. The judges are St. David, St. Teilo, St. Illtud, St. Dochau, St. Cynidr, St. Maeddog, and several other clerics and elders from all of Britain.
After much debate it is decided that Cadog may keep Ligessauc in safety in return for giving to Arthur a hundred cows for each of the three men that Ligessauc had slain. Arthur accepts, on the condition that all the cows given to him should be red-colored on their forequarters and white on their hindquarters.
Cadog calls for nine heifers, or in some accounts one hundred heifers, of any number, to be brought before the judges. They pray to God and the heifers are miraculously transformed into the three hundred heifers required by the judgment, all colored as required. The heifer are driven into a ford on the river Usk to be received by Arthur and his men. But as soon a Kay and Bedwyr lay their hands on the heifers, the heifers are transformed into bundles of fern.
Seeing this miracle, Arthur begs Cadog for forgiveness. It is agreed that any refugee in Cadog’s territory is to have another seven years free from outside persecution. Any refugee who leaves, but who is driven by storm back to Cadog’s territory, has the right to live there, undisturbed, for the rest of his life.
The bundles of fern vanish and the cattle reappear in the byres of their original owners. Cadog bestows homesteads on three of the saints there. To St. David, Cadog gives Llanddewi Penn Bei, to St. Teilo he gives Merthyr Tegfedd, and to St. Doguuinus he gives Llanddyfrwyr.