Aeddan was supposedly the son of Caw, the great Pictish overlord of the North. He travelled south, perhaps with his brother Gildas and became a pupil of St. Dyfrig, then a friend of St. Dewi. Along with Teilo and Ysfael, Aeddan and Dewi travelled to Mynyw (St. Davids) where Dewi founded his famous abbey. They were at first harassed by an Irish pirate named Bwya but he was eventually struck down and his fortress burnt to the ground.
An old tale is often told of how, while Aeddan and Teilo were reading in the cloister at Mynyw (St. Davids), they were called upon to replenish the monastery's fuel stores. Annoyed at having been drawn away from their studies, the two monks took their axes off to the woods; but found their task much easier than expected when two tame stags aided them in carrying the wood home. There is a well and a pool, named after Aeddan, within two miles of St. Davids and he is patron of the church at Llawhaden in Pembrokeshire.
Later, Aeddan crossed the Irish Sea and founded the monastery of Guernin (Ferns) in the Emerald Isle, where he was known as Maeddog. He is said to have left his bell behind, which St. Dewi had given him, and found it miraculously transported to his side! Traditionally, he also helped halt the English invasions of Britain by the use of prayer. He is, unfortunately, widely confused with an early 7th century Bishop of Ferns of the same name and is sometimes, wrongly, called a Bishop of Llandaff.