There is only one person left who could have been the historical source of Arthur; Riothamus.

Riothamus was a High King in Britain in the 5 th century. The first time he appears is in the Gothic History by a certain Jordanes. There is told that the West emperor sought for a British alliance against the Saxons with their king Riothamus. Who is according to professor James Campbell, acceptable as “a British ruler having authority on both sides of the Channel”. Riothamus fights against the Visigoths, but his army loses and he gets lost in history.

This story can tell us a lot about the similarity's between Arthur and Riothamus:

  1. Riothamus led an army of Britons into Gaul, and was the only British King who did (just like Arthur, according to the stories).
  2. Riothamus was betrayed by a deputy ruler who cooperated with barbarian enemies (just like Arthur was betrayed by Modred).
  3. Riothamus disappears after a fatal battle, without any recorded death. His departing shows that he is going to a town called Avallon (Arthur also left to Avalon after his final battle)

Geoffrey of Monmouth was the first one to talk about the expedition to Gaul, the betrayal of Modred and Arthur leaving to Avalon. It is most likely that Geoffrey got this ideas from a source which tells about Riothamus.

However, there are a few problems with this theory. The name is the first problem, he isn't called Arthur. The name Riothamus could be explained by the fact that a lot of people had two names in that time. A Romain and a Britain one. Riothamus probably would have been Rigotamos. The first part of the would mean “king” if taken as noun, “kingly” or “royal” if taken as an adjective. The second part is a superlative ending like “-est” in English, as in words like “kindest”. So Rigotomas might mean the “king-most” or “supreme king”. If that is the case, Arthur could have been the British name of Riothamus.

The second problem is the date. According to the timescale of Nennius, Arthurs reign was at the end of the 5 th century. Riothamus died in 470, so if the dating is correct he can't be Arthur after all. Nevertheless, Ashe can prove that Arthur's reign was before 470. He made a timescale, based on the names which Monmouth used in his story, during the expedition to Gaul:

The story was set during the last period of the Western Roman Empire, this dates from 455 and 476. The story also speaks of the Emperor of the East: Leo. This cuts the range to 457 to 474. There is also evidence that the story is set when Simplicius was Pope. He does overlap Leo, but the overlap is confined to six years; 468-474. According to Ashe there are grounds for thinking that they happened when “Lucerius” is said by a chronicle have been Emperor in the West. The years in question lie within Leo-Simplicius overlap. They are 469-470. The last person who could be important, is a bonus. A fourth name came up; Childeric. Childeric was a Frankish King whose pact with the Saxons preserved their remnants in the Loire Valley. In this case Geoffrey would have got a name only, but he could have got that, plus a vague association with the Saxons and double-dealing; and Childeric; in the form Cheldric; is the name he gives to the Saxon overlord with whom Modred makes his treacherous treaty. But in this case, we are not dealing with history, but with fiction.

Dating Arthurs expidition to Gaul in 469-470 places the expedition at the end of Riothamus' life. This could be correct, because Riothamus disappeared after his last fight in Gaul. The left on a course, which led to the French Avallon. Placing Arthurs war in Gaul in 469 and 470, makes it more believable that Riothamus was indeed the historical source behind Arthur.

We can use another source to prove that Arthur was originally placed in the lifetime of Riothamus. In the Lation story “Goeznovius”, written by a man named William –which is not written with Monmouths History of the Kings of Britain as a source – is spoken about almost the same names and happenings as in History of Kings of Britains. However William places Arthur right after Vortigern. He doesn't speak about Ambrosius or Uther. He also says that the Saxons came to the Island of Britain again, after Arthur's death. This must be in the late 460's according to historical sources about the invasions of the Saxons.

William also places Arthur in the time in which he could have fought most of Nennius battles in Britain.

There is another source: A Cistercian monk, Alberic, compiled a year-by-year chronicle based chiefly on French information. He places Arthur's reigning in 459 and his death in 470. On that reading he could be Riothamus.

According to all this information, the period in which King Arthur lived, must have been the same as in which Riothamus lived.

Another problem is the enemy. In Geoffrey's History of Kings of Britain Arthur fights against the Romans, but Riothamus and the Romans were allies. It is not new that in heroic stories the enemies change and even in Geoffrey's story you can find pieces of another enemy. Somewhere in the story some of the sub kings speak to go over sea, to fight the Romans and the Germans. Those Germans were probably the Goths and the Saxons. Geoffrey forgets them, because they there are no longer needed for his story. The only reason why they were added in the first place, was probably because his historical source spoke about a war against the Germans, Riothamus.

When you put all of this together you can say that we found the largest historical source behind Arthur: Riothamus. 1