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Lucius Tiberius (sometimes Lucius Hiberius, or just simply Lucius) is a fictional Roman Emperor from Arthurian legend appearing first in Geoffrey of Monmouth's Historia Regum Britanniae. No Roman Emperor of that name ever existed; Geoffrey either heard of him from folk tradition or made him up. Geoffrey Ashe theorizes that he was originally Glycerius, whose name was known to have been misspelled as "Lucerius" in texts prior to the writing of the Historia Regum Britanniae, and was further misspelled by Geoffrey of Monmouth. He may also have been based on Tiberius II Constantine who attempted to re-establish Roman hegemony in the West.
Lucius appears in later, particularly English literature such as Thomas Malory's Le Morte d'Arthur, and the Alliterative Morte Arthure, and a Roman Emperor defeated by King Arthur appears in French Arthurian literature as well, notably in the Vulgate Cycle.
Lucius fights to reclaim Gaul from his treacherous tribune Frollo at the time of King Arthur's rise to power. After Arthur conquers Gaul, word of his great deeds reaches Rome, and Lucius demands that Arthur pay him tribute and recognize him as his sovereign. When Arthur refuses, Lucius invades the land of Arthur's allies on the continent, and Arthur and his knights hurry across the English Channel to do battle with him. Arthur defeats Lucius and adds Italy to his lands.