'The vessel is inscribed, telling us who Sedatus was: a Roman citizen (because of his triple name) and the presiding ritualist who summoned the spirits. Beneath this statement is a long list of spirit names, almost all of which are unknown to archaeologists. But one stands out: ‘Dru’. If we are right in assuming this is an abbreviation of ‘Druid’ (and what else could it be?) then it is the only direct archaeological evidence for the existence of the Druids.'

https://aeon.co/essays/what-can-archaeology-tell-us-about-the-druids-dark-arts