Riothamus Riothamus ( also spelled Riutimus or Riotimus ) was a Romano - British military leader , who was active circa AD 470 . He fought against the Goths in alliance with the declining Roman Empire . He is called " King of the Britons " by the 6th - century historian Jordanes , but the extent of his realm is unclear . Riothamus is a Latinization of the Brythonic personal name " * Rigotamos " , meaning ' king - most ' , ' supreme king ' or ' highest king ' . Though it is still a matter of debate , several scholars consider his life to have been one of the possible sources for the King Arthur legend . Realm . It is not clear whether Jordanes ' " Britons " refers to the Britons of Great Britain itself , or of Armorica , which was undergoing significant British settlement and later came to be known as Brittany . The Bretons retained strong links to Britain , as is reflected in the names Kernow ( now Cornwall ) and Dumnonia ( now Devon ) being found as Cornouaille and Domnonée in Armorica . The distinction between insular and continental Britons may not have had very much meaning at the time , as ecclesiastics such as St . Winwaloe were associated with Brittany and Great Britain alike , and King Mark ( ' Hound of the Sea ' ) apparently ruled Britons / Bretons on both sides of the English Channel . The Old Breton name " Riatam " , which ( like Riothamus ) is derived from Brittonic " * Rigotamos " , appears in medieval Breton records ( primarily biographies of early Breton saints ) as one of the Princes of Domnonée ( a coastal region in Brittany which takes its name from Dumnonia in southern Britain ) . He is identified as a son of Deroch II . For chronological reasons , this Riatam is probably a different individual from Jordanes ' Riothamus , who lived earlier . To resolve this contradiction , some scholars have speculated that the name Deroch II may be an error for the earlier Deroch I , which would make " Riatam " contemporary with Jordanes ' Riothamus . The " Riatam " of Domnonée is said to have been exiled in Britain after his father ' s death , apparently during a civil war . According to chronicles , he returned to kill the usurper . Correspondence with Sidonius Apollinaris . More secure information is provided by a letter which has survived that was written to Riothamus from Sidonius Apollinaris , bishop of Clermont , who requested his judgment for " an obscure and humble person " who had had his slaves enticed away by a group of armed Bretons . According to C . E . V . Nixon , the letter is evidence that Armorica at this time was becoming " like a magnet to peasants , " coloni " , slaves and the hard - pressed " as Roman power weakened . Poorer subjects of Rome with no stake in land ownership found Breton territory to be a safe haven from the Goths . War with the Goths . Jordanes states that Riothamus supported the Romans against the Visigoths led by Euric ( who lived c . 440 – 484 ) . In " The Origin and Deeds of the Goths " , he states that Riothamus brought a British army to supplement Roman forces but suffered defeat fighting against overwhelming odds when the Goths intercepted his force : A letter from Sidonius Apollinaris records that the Praetorian Prefect of Gaul Arvandus sent a message to the Gothic king Euric stating that " the Britons stationed beyond the Loire should be attacked " , which has led some to suggest that Arvandus betrayed Riothamus . This letter does not mention Riothamus by name , however , and ( depending on the reconstruction of the chronology of Sidonius ' letters ) , it is possible that Riothamus and his forces were not the direct subject of Arvandus ' message to Euric , as Arvandus might have already been under arrest and on his way to Rome before Riothamus had even entered the fray against the Goths . Gregory of Tours seems to react to the outcome of the battle between the Goths and Britons : " " Brittani de Bituricas a Gothis expulsi sunt , multis apud Dolesim vicum peremptis " " ( The Brittani were driven from Bourges by the Goths and many of them perished at the village of Déols ) . King Arthur . Riothamus has been identified as a candidate for the historical King Arthur by some recent writers and scholars , notably the academic Léon Fleuriot and popular historian Geoffrey Ashe . These authors further note that Riothamus ' last known position was near the Burgundian town of Avallon , which they suggest is the basis for the Arthurian connection to Avalon . In any case , Riothamus ' activities in Gaul may be the seed whence grew the tradition ( first recorded by Geoffrey of Monmouth in his " Historia Regum Britanniae " ) that Arthur crossed the English Channel from Britain and attacked Rome . Geoffrey Ashe has also suggested a link between Riothamus ' betrayal by Arvandus and Arthur ' s betrayal by Mordred in the " Historia Regum Britanniae " . Léon Fleuriot argued that Riothamus is identical to Ambrosius Aurelianus , a figure in early narratives about the period when Arthur is supposed to have lived . He suggests that " Riothamus " was Aurelianus ' title as overlord of all Brythonic territories . He noted that " Riothamus " and Aurelianus are contemporaneous and that Aurelianus is the only British leader of the time who is identified as ruling both Brythons and Franks , which could only be the case if he ruled territory in Brittany . He also suggested that the name " Amros " in Breton genealogies is a contraction of " Ambrosius " and that Nennius refers to Aurelianus as supreme ruler of the Britons , which would translate as " Riothamus " . Fleuriot argued that Ambrosius led the Britons in the battle against the Goths , but then returned to Britain to continue the war against the Saxons .