[1] According to many hagiographical accounts, including those of Gildas, Bede, Geoffrey of Monmouth, and Matthew of Paris, Amphibalus was a Roman Christian fleeing religious persecution under Emperor Diocletian.

[2][3] Amphibalus gained his name and title[1] when Geoffrey of Monmouth wrote his Historia Regum Britanniae (History of the Kings of Britain) in the 12th-century.

Wilhelm Levison[5] noted that the story of the name, which goes back to a 5th-century Passio Albani, is composed of borrowings from other lives of saints and it has, in his words, "no place in the ranks of Acta martyrum sincera; it is a legendary tale...." Geoffrey repeated the story of Alban's martyrdom as given by Bede in his famous Historia Regum Britanniae[6] (c. 1136), with the addition of the name of the confessor he shelters, Amphibalus.

[9] Geoffrey may have gotten the name from Gildas, who describes his contemporary, Constantine, King of Dumnonia, as having dressed in the amphibalo, or 'cloak', of an abbot to murder two royal youths in a church.

He provided an elaborate version of the story of Saint Alban and gave a prominent role in it to a new martyr-saint, Amphibalus, whose name he states to have found in Geoffrey's work.

[11] Wilhelm Levison,[12] stated that: "The abbey had incurred heavy debts; anyone who knows the medieval misuse of pious belief and offering, will not be surprised to learn that just at this time the generosity of the devotees was stimulated by the discovery of the history and, what is more, of the relics of St Amphibalus.

"[17] This phenomenon bears witness to the influence of Geoffrey of Monmouth's Historia Regum at the time which lay behind the discovery of the grave of King Arthur at Glastonbury Abbey.

This was around the same time that the bodies of Amphibalus and his companions were discovered, which Gordon-Taylor[18] suggests was also motivated in part by competition with the new Canterbury cult of St Thomas Becket to gain pilgrims.

[20] After the martyrdom of Alban, Amphibalus was believed to have returned to Caerleon, where he converted many others to Christianity, including the saints Julius and Aaron.

[4] In 1178, some 800 years after Amphibalus' traditional death date, his remains were discovered at Redbourn in Hertfordshire, England, near the town of St Albans.

According to the tale, Saint Alban appeared in a vision to a monk named Robert, indicating that he wished to make the location of the remains of Amphibalus known.