Historical basis for King Arthur

Riothamus

Riothamus (aka Riotimus, apparently meaning Kingliest or "Great King" (from Brittonic *rigo- "king", plus the Brittonic superlative suffix -tamo-) was a historical figure whom ancient sources list as "a king of the Britons". He lived in the late 5th century, and most of the stories about him were recorded in the Byzantine historian Jordanes' The Origin and Deeds of the Goths, written in the mid-6th century, only about 80 years after his presumed death.

About 460, the Roman diplomat and bishop Sidonius Apollinaris sent a letter to Riothamus, asking his help to quell unrest among the Brettones, British colonists living in Armorica. This letter still survives.

In the year 470, the Western Emperor Anthemius began a campaign against Euric, king of the Visigoths who were campaigning outside their territory in Gaul. Anthemius requested help from Riothamus, and Jordanes writes that he crossed the ocean into Gaul with 12,000 warriors. The location of Riothamus’s army was betrayed to the Visigoths by the jealous praetorian prefect of Gaul, and Euric defeated him in a battle in Burgundy. Riothamus was last seen retreating near a town called Avallon.[citation needed]

Geoffrey Ashe points out that, as above, Arthur is said by the early sources to have crossed into Gaul twice, once to help a Roman emperor and once to subdue a civil war. Assuming that Riothamus was a king in Britain as well as Armorica, he did both. Arthur is also said to have been betrayed by one of his advisors, and Riothamus was betrayed by one of his supposed allies. Finally, it is well known how King Arthur was carried off to Avalon (which Geoffrey of Monmouth spells "Avallon") before he died; Riothamus, escaping death, was last known to have been in the vicinity of a town called Avallon.

It is unknown whether Riothamus was a king in Britain or of Armorica; as Armorica was a British colony and Jordanes writes that Riothamus "crossed the ocean", it is possible both are correct. The name Riothamus has been interpreted by some as a title "High King", though there is no evidence for such a title being used by ancient Britons or Gauls and the formation of the name (noun/adjective + superlative -tamo- suffix) follows a pattern found in numerous other Brittonic and Gaulish personal names (for example, Old Breton/Welsh Cunatam/Cunotami/Condam/Cyndaf [Brittonic *Cunotamos "Great Dog"], Old Welsh Caurdaf [Brittonic *Cauarotamos "Great Giant"], Old Welsh/Breton Eudaf/Outham [Brittonic *Auitamos "Great Will/Desire"], Uuoratam/Gwrdaf [Brittonic *Uortamos "Supreme"], Old Breton Rumatam [Brittonic *Roimmotamos "Great Band/Host"], Gwyndaf [Brittonic *Uindotamos "Fairest/Whitest/Holiest One"], Breton Uuentamau [Brittonic *Uenutamaua: "Friendliest or *Uendutamaua: "Little Fairest/Whitest/Holiest (One)"]).[41]

Cognates of the name Riothamus survive in Old Welsh (Riatav/Riadaf) and Old Breton (Riatam).
Ambrosius Aurelianus

Ambrosius Aurelianus (also sometimes referred to as Aurelius Ambrosius) was a powerful Romano-British leader in Britain. He was renowned for his campaigns against the Saxons, and there is some speculation that he may have commanded the British forces at the Battle of Badon Hill. At any rate, the battle was a clear continuation of his efforts.
Artúr mac Áedáin and others

Though he was the eldest son of Áedán mac Gabráin, Artúr mac Áedáin never became king of Dál Riata; his brother Eochaid Buide ruled after his father's death. However, when Áedán gave up his role and retired to monastic life, Artúr became war leader, though Áedán was officially still king. Thus it was Artúr who led the Scotti of Dál Riata in a war against the Picts, separate from the later war with Northumbria. By this theory, Artúr was predominantly active in the region between the Roman walls — the Kingdom of the Gododdin. He was ultimately killed in battle in 582. This is the solution proposed by Michael Wood.[42]