Riothamus is the style given in continental documents to a fifth-century "king of the Britons", whose career seemingly underlies parts of the account of Arthur in Geoffrey of Monmouth. It Latinizes a title or honorific in the British language - that is, the parent tounge from which Welsh, Cornish, and Breton evolved.
The known context is the disintegrating Roman Gaul of the late 460s. The north was dominated by Syagrius, a man of imperial loyalty ruling a fragment of the Empire. Within his sphere of influence were Franks who cooperated with him, and lately arrived Britons in Armorica, the pioneer Bretons. Southward were the Burgundians, with rulers of their own but friendly to Rome. Hostile Saxons, however, were settled along the lower Loire, and a serious threat came from the southwest, where the Visigothic king Euric was powerful on both sides of the Pyrenees and had ambitious designs.
In 467, the eastern emperor Leo I appointed Anthemius, a Byzantine noble, as his western colleague. A pressing need was to check Euric. Britain had been out of the Empire for decades, but Anthemius, possibly encouraged by a pro-Roman trend among its rulers, negotiated a British alliance. The "king of the Britons", Riothamus, came to Gaul in 468 "by the way of Ocean" with 12.000 ship-borne troops (thus the sixth-century historian Jordanes). The Britons were temporarily stationed north of the Loire and may have assisted in a campaign against the Saxons near Angers. But Gaul's imperial prefect, Arvandus, was acting treacherously. He wrote to Euric urging him not to come to terms with the "Greek emperor", meaning Anthemius, but to crush "the Britons posted north of the Loire" and carve up Gaul with the Burgundians. Arvandus was detected and brought to justice, but the Empire's hollowness in Gaul was exposed.
Meanwhile, Riothamus moved into Berry and occupied Bourges. Probably in this phase, the Gallo-Roman author Sidonius - formerly city prefect of Rome, now bishop of Clermont, or shortly to become so - wrote to him on behalf of a landowner whose slaves the Britons were enticing away, doubtless to employ them as bearers or mercenaries. Riothamus advanced to Déols near Châteauroux. There Euric pounced on him with an overwhelming force, "before the Romans [presumably the troops of Syagrius] could join him". The Britons were defeated and Euric drove them from Bourges. Riothamus escaped with a remnant of his army into the nearby territory of the friendly Burgundians, in late 469 or early 470.
Some historians have minimized him as simply a chief of Bretons - i.e., settlers in Armorica - but at the time the Armorican settlement was too sparse. The scattered colonies could not have fielded an army with any prospect of checking Euric; and Arvandus's proposal to him would have made little sense, since he would have achieved nothing by overrunning them. Jordane's testimony that the Britons came "by the way of Ocean" seems decisive. An army from Britain would have done so, the Armorican settlers would not, since they were on the Continent already and had been for years. A king of the Britons might have gathered recruits from them and counted them as still his subjects. In the judgment of James Campbell (1982), Riothamus is credible as "a British ruler having authority on both sides of the Channel". That he was prepared in 468 to take soldiers out of Britain, and stay overseas for at least a year, helps in dating Ambrosius Aurelianus. Action against the Saxons on the island, under Ambrosius's leadership, had apparently convinced its rulers that they were contained and neutralized.
The Latin form Riothamus, used with slight variants in continental texts, corresponds to the British Rigotamos. Rig, with the added o in a compound, meant "king"; tamos was a superlative suffix. As a noun, this word would mean "king-most" or "supreme king" (cf. the modern word generalissimo). As an adjective, it would mean "most kingly", "supremely royal". It appears later as a proper name, becoming Riatham in Breton and Rhiadaf in Welsh. But in the fifth-century setting - in Sidonius's letter, for instance - it looks like a designation for a ruler whose name was something else. Cognate in sense with Vortigern and Vortimer, and applied to a king of the Britons, it suggests that he was one of the high kings of the period - recasting the style, perhaps, to dissociate himself from discredited predecessors. To judge from the cross-Channel context, he probably held territory in the southwest, but at that time, when the Britons' unity under Rome was still remembered, he might have claimed paramountcy over a much wider area. At any rate, it is hard to believe that his parents happened to give him a name that turned out to be so apt for the office he held as an adult.
As a noun, "supreme king", Riothamu would be a title. Similarly, "Genghis Khan", meaning "very mighty ruler", is the accepted designation of a Mongol chieftain whose name was Temujin. As an adjective, "supremely royal", Riothamus would be a honorific. Again, the first Roman emperor, whose name was Octavian, adopted the epithet Augustus, meaning "majestic", with an overtone of sacredness; after which every emperor was Augustus, "His Majesty". Plutarch records a Greek word, "Basileutatos", exactly equivalent to Riothamus, which was definitely not a name but a term of honor bestowed on Minos of Crete.
That this king of the Britons would have had seperate name - and could have had another one even if Riothamus was his name - has inspired attempts to give him substance in Briton by identifying him with someone else. Fleuriot makes him out to be Ambrosius. One of several objections is that Gildas, the single real authority, describes Ambrosius as a general only. But Fleuriot acknowledges that the story of Riothamus, whoever he was, has gone into the making of the story of Arthur. Geoffrey of Monmouth portrays Arthur campaigning in Gaul and makes him contemporary with persons living in Riothamus's time, notably Leo I. Nor is he the only author who does so.
A Riothamus appears in Breton genealogies who may be the King, correctly or incorrectly dated. But there were certainly other men so called in the sixth century.