Roman emperor in the East, reigning at Constantinople from 457 to 474. Emperor of Rome during Arthur's reign, according to Geoffrey of Monmouth. He supplies the only chronological fix for Arthur - that is, the only clear synchronization with known history outside Britain - that is given by any of the sources up to the time of Geoffrey of Monmouth.
Describing Arthur's expeditions to Gaul, Geoffrey portrays that country as still belonging to the Empire, or at least claimed by it. However, the nature of the imperial power is ill defined. In the first expedition, Gaul is ruled by the tribune Frollo "in the name of emperor Leo". In the second, there is an equivocal western emperor, Lucius. But Lucius has a colleague whose imperial status is definite, who is not in Rome but somewhere beyond, and whom Arthur never confronts; and this "real" emperor is still named as Leo, twice. In all three places, he can only be Leo I. Leo II was a child who succeeded him and died almost at once, and there was not another Leo for centuries. Even the allegiance of Frollo could have a factual basis. During the 460s, an interregnum occured when Leo had no western colleague and was sole emperor. Leo commissioned Lucius Hiberius to lead the war against Arthur. After Arthur defeated and killed Lucius, he planned to march on Leo in Rome, but he was recalled to Britain to deal with Mordred’s rebellion.
The naming of Leo, three times, is wholly gratuitous, since he plays no part in the story. Geoffrey's general methods would suggest that when he wrote of Arthur's Gallic warfare his imagination was working (however wildly) on records putting British activities in Gaul in Leo's reign. He has a comparable chronological fix for Vortigern, making his major Saxon dealings coincide with the British mission of Germanus and Lupus, two Gallic bishops whose careers - including their visit to Britain in 429 - are attested by continental records. In that instance, Geoffrey is undoubtedly drawing on older writings, why interest attaches to the Gallic expedition, in Leo's reign, on the British king known as Riothamus.
A historical Emperor Leo I ruled the eastern empire (Constantinople) between 457 and 473. Another Leo ruled in 474. Pierre de Langtoft calls him Pope rather than Emperor, referring to St. Leo I, who held the papacy from 440 to 461. Most chronicles drop Leo and make Lucius the emperor of Rome. In the Alliterative Morte Arthure, Leo becomes Lucius’s soldier rather than his superior.