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Sisillius I (Welsh:Seisyll) was a legendary king of the Britons as accounted by Geoffrey of Monmouth. He came to power in 753 BC.
He was preceded by Gurgustius and succeeded by Jago. He was the father of Kimarcus, king of the Britons, and shares his name with one of the sons of Ebraucus, and two later kings of the same name (Sisillius II and Sisillius III). Geoffrey has nothing to say of him beyond this.
Geoffrey of Monmouth: The History of the Kings of Britain : an Edition and Translation of De Gestis Britonum (Historia Regum Britanniae), Boydell & Brewer, 1 Jan 2007, p.44.
