A strong anti-Christian movement in Strathclyde, headed by a certain King Morken, compelled Mungo to leave the district, and he retired to Wales, via Cumbria, staying for a time with Saint David at St David's, and afterwards moving on to Gwynedd where he founded a cathedral at Llanelwy (St Asaph in English). While there, he undertook a pilgrimage to Rome. However, the new King of Strathclyde, Riderch Hael, invited Mungo to return to his kingdom. He decided to go and appointed Saint Asaph/Asaff as Bishop of Llanelwy in his place.