Sisillius (Welsh: Saessyllt) was the name of three legendary Kings of the Britons as accounted by Geoffrey of Monmouth: Sisillius, one of the younger sons of Ebraucus Sisillius I, successor of King Gurgustius Sisillius II, son of King Guithelin and Queen Marcia Sisillius III, successor of King Oenus British traditional history