In the late 5th century the Visigoth kingdom in what is now western France, had deployed a substantial fleet in the Bay of Biscay. When the kingdom was conquered by Clovis the Frank ( Claudas in Malory) in 507 AD the survivors were pushed into Mediterranean Spain. The fleet lost its Atlantic harbours. 

There is a tradition that "When Clovis the Frank deprived the Visigoths of their Atlantic harbours their military ships, under command of Theoderic, son of Theudebald sought sanctuary and employment elsewhere." 

Theoderic couldn't have been Christian although according to Paul Johnson - A History of Christianity -  the Visigoths were Arian Christians

Theoderic's name is named as one of Arthur's captains in the south - west. Theoderic is a uniquely Gothic name, not British, not Roman, not English; Gothic. Theoderic is known to have fought against the Irish in West Wales. In several Cornish histories Theoderic is named as a sub-king of Dumnonia. Theoderic is named in welsh sources as having recovered Southern Wales from the Irish. His name is mentioned in connection with the westernmost extremities of both Wales and Cornwall,  suggesting a maritime strength. A short sea crossing or a land journey of several hundred miles.

Theoderic was the overlord of what is now western Cornwall. He had two principal residences in the west. one near St.Ives on the North coast the other on the south near Falmouth. The ruins of both can still be seen.

When an Irish invasion, or migration, of 770 men landed at Hayle Bay in North Cornwall they were defeated in battle by Theodoric. The surviving Irish settlers / invaders were absorbed into the local communities. As Christian martyrs, their names are commemorated in parish church dedications all over south Cornwall.

 Malory renames Theoderic as King Idres. 

 Theodoric is recorded as having met his death in a hunting accident sometime between 530 and 550 AD.