Tathan was a Celtic missionary and theologian who lived during the fifth or sixth century. Originally from Ireland, he traveled to Wales, where he is recognized as an early abbot of Caerwent. According to historical accounts, he chose a life of spiritual devotion over inheriting his father's throne, traveling to Britain with eight followers. He established a monastic school at Venta Silurum that attracted students from various regions, including the young Cadoc.