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.
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.
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: Euricus ergo, Vesegotharum rex, crebram mutationem Romanorum principum cernens Gallias suo iure nisus est occupare.
Quorum rex Riotimus cum duodecim milia veniens in Beturigas civitate Oceano e navibus egresso susceptus est.
Vad quos rex Vesegotharum Eurichus innumerum ductans advenit exercitum diuque pugnans Riutimum Brittonum rege, antequam Romani in eius societate coniungerentur, effugavit.
Qui amplam partem exercitus amissam cum quibus potuit fugiens ad Burgundzonum gentem vicinam Romanisque in eo tempore foederatam advenit.
Eurichus vero rex Vesegotharum Arevernam Galliae civitatem occupavit Anthemio principe iam defuncto....[6] "Now Euric, king of the Visigoths, perceived the frequent change of Roman Emperors and strove to hold Gaul by his own right.
"[7] A letter from Sidonius Apollinaris to his friend Vincentius, written circa 468 AD, records that Roman officials intercepted a letter written by the Praetorian Prefect of Gaul, Arvandus, to the Visigothic king Euric stating that "the Britons stationed beyond the Loire should be attacked" and that the Visigoths and Burgundians (who were at the time clients of the Romans) should divide Gaul between them; this has led some scholars (such as Geoffrey Ashe) to suggest that Arvandus betrayed Riothamus.
Veniente vero Adovacrio Andecavus, Childericus rex sequenti die advenit, interemptoque Paulo comite, civitatem obtinuit.
In a great fire on that day the house of the bishop was burned.Riothamus has been identified as a candidate for the historical King Arthur by several scholars over the centuries, notably the historian Geoffrey Ashe,[11][12] primarily due to Riothamus's activities in Gaul, which bear a casual resemblance to King Arthur's Gallic campaign as first recorded by Geoffrey of Monmouth in his Historia Regum Britanniae.
[13] Fleuriot suggested that Ambrosius led the Britons in the battle against the Goths, but then returned to Britain to continue the war against the Saxons.