Commemorated on November 6
According to Life of St. Illtud written circa 1140, Illtud was the son of a Breton prince and a cousin of King Arthur. According to this Life, Illtud's parents intended him for service in the church and had him educated in literature for this purpose. However, he forsook his religious upbringing, choosing instead to pursue a military career. He took a wife named Trynihid, and became a soldier in western Britain (now Wales), in service first to King Arthur, and then to the King Poulentus. As a result of this, he is sometimes called Illtud the Knight. One afternoon, he took a hunting party onto the lands of Cadoc. The party sent a message to the abbot, demanding that the abbot feed them. The abbot deemed their demand to be very rude and improper, but graciously offered them a meal anyway. Before they could enjoy the meal, the ground opened up and swallowed the whole party as just punishment for their impiety. Only Illtud was spared, and he went to St. Cadog on his knees, begging forgiveness for his sinful act. The abbot told him to give up his selfish ways and go back to his religious upbringing. Inspired, Illtud gave up his wife, and became a hermit in the Vale of Glamorgan (a matrimonial detail which was regarded as dubious).
Illtud helped pioneer the monastic life of Wales by founding a monastery in what is now Llantwit Major. This became the first major Welsh monastic school, and was a hub of Celtic Christianity in Sub-Roman Britain. Illtud's own pupils are reckoned to have included seven sons of British princes and scholars such as Saint Patrick, Paul Aurelian, Taliesin, Gildas and Samson of Dol. David is also believed to have spent some time there.