Riothamus is considered by some scholars an epitaph or title rather than a personal name, probably derived from British Rigotamos, "King-most" "High King". He was a ruler in both Britain and Brittany. Riothamus was one of the British kings that led an army of Britons into Gaul and as Geoffrey Ashe points out, this fact matches Arthur's continental campaigns and therefore makes him a candidate for the historical Arthur. 
   In Gaul, Sidonius of Clermont-Ferrand wrote to Riothamus, to seek redress for a friend of his against a substantial British landowner. His letter indicates that he knew something of the character of Riothamus from previous correspondence or meetings in which they had mutual acquaintances.
   Gregory of Tours places Riothamus in 6th century Vannes, one of the holdings of Riothamus' father, Cynan Meriadauc, leader of the first migration from Britain to Brittany according to John Morris in his The Age of Arthur, A History of the British Isles from 350 to 650. In the same chapter of Morris' book, Riothamus is "said to have been" founder of the dynasties of eastern Domnonie, and his son, Daniel Dremrud, is the 'king of the Alamanni'.
   A possible historical timeline for the continental campaign of Riothamus is as follows: The Goths were settled in southern Gaul by treaty with the Roman commander and future emperor Constantius III in 415. After the death of Honorius, Galla Placidia, Valentinian's mother, ruled as Regent of the Western Empire from 423 until 450, when Valentinian assumed the throne. Aetius, who was the mainstay of the Western Roman Empire during this period, was murdered by Valentinian III in 454. Aetius is the same individual to whom the plea from Britain was made circa 446. Valentinian was murdered the following year in 455. A great number of historians list the dates between 455 and 467 as the disintegration of the Western empire, with several usurpers claiming the throne in this interval. In 466, Euric became the King of the Visigoth. In 467, Anthemius was appointed as the Emperor of the West by Leo I of the Eastern Empire. Anthemius' rule lasted from 467 to 472. During this period when Avitus's son in law, Sidonius, was prefect, there were two trials of Romans for treason which relate to a proposal to have Euric take over southern Gaul to end the weakness and ineffectiveness of imperial rule there. Also at this time, Jordanes and Gregory of Tours write of the advent of the Briton Riothamus from across the ocean. He sailed with a fleet carrying 12,000 men into the interior via the Loire. For his assistance, the Bretons under Riothamus were given the estates north of the Loire and the Armorican Tract. But he was betrayed, utterly routed by Euric, and retreated into Burgundy toward Avallon. Riothamus is not mentioned in history again. In 472, Euric withdrew from the treaty of 415 and by 475 had conquered all of southern Gaul except Savoy which was held by the Burgundians and the Auvergne which was held by Ecdicius, son of Avitus. In 475, the bishop of Marseille engineered a swap of territory under which Auvergne was swapped for Provence, in order to protect Italy. The scandal that this provoked led to the collapse of the regime of Julius Nepos and the usurpation of the father of Romulus Augustulus, who used his son as a puppet. In 476, Euric seized Provence anyway and the army used the opportunity to abolish the post of western Emperor and place itself at the disposal of the eastern emperor by returning the imperial regalia to Constantinople. Letters of Sidonius tells us that Euric's court, in Bordeaux, was staffed mainly by Romans, whom he writes to by name.
   Riothamus is recorded by Jordanes as fighting for the Empire against the Visigoths on the Continent. By trying to identify various personages mentioned in Geoffrey's Historia about Arthur - namely, the Emperor Leo - Leo I, Lucius Hiberus - a mistake for Sigebert's Lucerius, and a Pope Sulpicius - possibly a corruption of Pope Simplicius reigning at the same time as Leo, Geoffrey Ashe equated him to Arthur and stated this overseas campaign occurred in the years 469-470CE. Most of the assumptions and identifications have been debated. As Dan Hunt has pointed out, "Another source utilized by Ashe to confirm his Riothamus theory is William's early 11th century Life of Saint Goeznovius (Gwyddno), which mentions Arthur as winning many victories "gloriously in Britain and Gaul". But Gaul and Wales could be confused, the two ancient spellings for these countries being identical in Latin and French. Gaels, i.e. Irishman, could even be confused for Gauls. Thus William's testimony cannot be relied upon to prove that Arthur was fighting in Gaul. Jordanes may very well have confused Brittany for Britain and Bretons for Britons, assuming as he did so that Britons from Britain must have come to Gaul "by the way of Ocean". Similar problems occur with Hiberus which can be Hiberus referring to Spain or a Spaniard or Hiber(n)us, a Gwyddel or Irishman. A mythological solution to the "Hiberus" problem was long ago proposed by Loomis: Procurator Lucius Hiber(n)us, derives from the Welsh hero Llenlleawg Gwyddel. He proposed that Llenlleawg itself was an epithet of the god Lugh and later became the name Lancelot, which is why in the later romances Lancelot plays the role previously played by Lucius Hiber(n)us.
   As for Avallon, John Morris on page 138 of his book,  says: "The romantic Avallon is a common Roman Celtic place name; it is still the name of one small town in central France, that was known as Aballo in the Roman period. The letters 'Avallon' painted upon a dusty board on the railway station are a healthy reminder of its prosaic reality, for it means no more than 'Appleton'. "
   John Morris was of the opinion that Riothamus was same person as the Riothamus in the genealogy of the Annals of Clonmacnoise, a John Reith, also called Regula and Riatham. He "defeated the 'keels' of five chiefs, and cut off their heads . . .Let my witness be the river Loire, by whose banks so many battles were then so keenly fought."
   Equally as interesting, Geoffrey Ashe points out that stories began popping up about Arthur during this period, and in some of the later stories, the date of 475 is ascribed as the year of Arthur's coronation. Just a series of coincidental dates? Certainly, Riothamus is a probable model for the continental campaign that Geoffrey of Monmouth ascribes to Arthur. Was Riothamus wounded to the death and disappeared to Avallon just as Arthur? Or could it be that since he came from across the ocean, he went back there, back to Britain and resumed his role as war leader.

Sidonius Letter c. 472CE

Sidonius Riothamo suo salutem.

1. Servatur nostri consuetudo sermonis: namque miscemus cum salutatione querimoniam, non omnino huic rei studentes, ut stilus noster sit officiosus in titulis, asper in paginis, sed quod ea semper eveniunt, de quibus loci mei aut ordinis hominem constat inconciliari, si loquatur, peccare, si taceat. sed et ipsi sarcinam vestri pudoris inspicimus, cuius haec semper verecundia fuit, ut pro culpis erubesceretis alienis.

2. gerulus epistularum humilis obscurus despicabilisque etiam usque ad damnum innocentis ignaviae mancipia sua Britannis clam sollicitantibus abducta deplorat. incertum mihi est an sit certa causatio; sed si inter coram positos aequanimiter obiecta discingitis, arbitror hunc laboriosum posse probare quod obicit, si tamen inter argutos armatos tumultuosos, virtute numero contubernio contumaces, poterit ex aequo et bono solus inermis, abiectus rusticus, peregrinus pauper audiri. vale.

A translation from http://www.tertullian.org/fathers/sidonius_letters_03book3.htm

To his friend Riothamus

1. I will write once more in my usual strain, mingling compliment with grievance. Not that I at all desire to follow up the first words of greeting with disagreeable subjects, but things seem to be always happening which a man of my order and in my position can neither mention without unpleasantness, nor pass over without neglect of duty. Yet I do my best to remember the burdensome and delicate sense of honour which makes you so ready to blush for others' faults.

2. The bearer of this is an obscure and humble person, so harmless, insignificant, and helpless that he seems to invite his own discomfiture; his grievance is that the Bretons are secretly enticing his slaves away. Whether his indictment is a true one, I cannot say; but if you can only confront the parties and decide the matter on its merits, I think the unfortunate man may be able to make good his charge, if indeed a stranger from the country unarmed, abject and impecunious to boot, has ever a chance of a fair or kindly hearing against adversaries with all the advantages he lacks, arms, astuteness, turbulences, and the aggressive spirit of men backed by numerous friends. Farewell.