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. According to Paul Johnson - A History of Christianity -  the Visigoths were Arian Christians

Theoderic is a uniquely Gothic name, not British, not Roman, not English; Gothic. (Theoderic is  a version used by the Romans of a germanic phrase meaning 'ruler of the people' )

Theoderic's name is named as one of Arthur's captains in the south - west;. Sir Thomas Malory renames Theoderic as King Idres in his Le Morte d'Arthur.

 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.

In several Cornish histories Theoderic is named as a sub-king of Dumnonia. He 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.

 "RIVIERE, near Hayle, now called Rovier, was the palace of Theodore, the king, to whom Cornwall appears to have been indebted for many of its saints. This Christian king, when the pagan people sought to destroy the first missionaries, gave the saints shelter in his palace, St Breca, St Iva, St Burianna, and many others, are said to have made Riviere their residence. It is not a little curious to find traditions existing, as it were, in a state of suspension between opinions. I have heard it said that there was a church at Rovier--that there was once a great palace there; and again, that Castle Cayle was one vast fortified place, and Rovier another. Mr Davies Gilbert quotes Whitaker on this point - "Mr Whitaker, who captivates every reader by the brilliancy of his style, and astonishes by the extent of his multifarious reading, draws, however, without reserve, on his fertile imagination, for whatever facts may be requisite to construct the fabric of a theory. He has made Riviere the palace and residence of Theodore, a sovereign prince of Cornwall, and conducts St Breca, St Ira, with several companions, not only into Hayle and to this palace, after their voyage from Ireland, but fixes the time of their arrival so exactly, as to make it take place in the night. In recent times the name of Riviere, which had been lost in the common pronunciation, Rovier, has revived in a very excellent house built by Mr Edwards on the farm, which he completed in 1791."

From - Popular Romances of the West of England collected and edited by Robert Hunt [1903, 3rd edition]

Theoderic is recorded as having fought against the Irish in West Wales. In one case local tradition tell 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.

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