About 460, the Roman diplomat and bishop Sidonius Apollinaris sent a letter to Riothamus asking his help to quell unrest among the Brettones, a colony of Brittonic-speaking people living in Armorica; this letter still survives. In the year 470, the Western Roman 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 soldiers into the land of the Bituriges, likely to Avaricum (Bourges). The location of Riothamus' army was betrayed to the Visigoths by Arvandus, the jealous praetorian prefect of Gaul, and Euric defeated him in the Battle of Déols. Riothamus was last seen retreating northwest to Burgundy.