Through his missionary work, he was able to successfully convert Gosbert, Duke of Wurzburg. The Duke had married Geilana, who was opposed to the Christian faith. She had previously beheaded three missionaries. Despite the stubborn beliefs of Geilana, the missionaries were able to convert Gosbert.
He ultimately became a martyr for the Church, by some accounts killed at the direction of Geilana, wife of Gosbert. Saint Killian is the patron saint of the diocese. In art Saint Killian is frequently shown as a Bishop with a sword in hand. The Feast Day of Saint Killian is celebrated on July 8.