The first mention of the cathedral dates back to 465, when a council took places in Vannes on the occasion of saint Patern’s consecration. Patern? One of the 7 saints founders of Brittany, with, among others, Corentin in Quimper and Tugdual in Tréguier. Patern came from Wales and left his native country to go in Brittany.
Inhabitants there appointed him bishop of Vannes. The very first bishop of Vannes, whoa! A prestigious job he get thanks to his pal Caradoc, a local leader from Vannes and famous knight of the Round Table... But a few people started to moan about Patern.
Hey, bushy-haired pagans, who didn’t want Christianity on their lands, expelled him! But later, they regretted... So Patern’s relics were brought back in Vannes. But, the legend (according to the book Guide de la Bretagne mystérieuse, ed Tchou) says the bearers carrying the sarcophagus could not lift it up!
Well, well, well: the last remains weighed heavy... Why? Bearers had to give money to the cathedral! Those relics stayed in Vannes until the Norman invasions in the 9th century. After that, they went in Central France, with a translation in Issoudun, until the 11th century. A part was translated in Vannes, then. So, we still can see relics in Saint-Pierre cathedral’s treasure!