The Historia Brittonum gives the following list for the kings of the Deira:
“Theodoric, son of Ida, reigned seven years. Freothwulf reigned six years. In whose time the kingdom of Kent, by the mission of Gregory, received baptism. Hussa reigned seven years. Against him fought four kings, Urien, and Ryderthen, and Guallauc, and Morcant. Theodoric fought bravely, together with his sons, against that Urien.”
The Flores Historiarum states:
“In the year of grace 570, Frethwulf reigned in Bernicia seven years. In this year the people of Armenia embraced the faith of Christ …
In the year of grace 577 … This year died Frethwulf, king of Bernicia, and was succeeded by Theodoric, who reigned seven years.”
The inversion in HB’s sequence, Theodoric followed by Freothwulf, in the FH’s list may be explained by the Chronicon ex Chronicis which gives a sequence of kings together with the lengths of their reigns:
“… Theodwlf uno, Freothulf VII., Theodric VII. …”
indicating the first name listed above was mistakenly written as Theodoric.
Freothwulf name becomes Frollo (Flollo in the Latin text) in the Historia Regum Britanniae. It takes the form Freol in The Awntyrs of Arthur. Indeed, in the Vulgate Merlin and Lancelot Frollo is said to be from Germany. Frollo’s flight to Paris may be a garbled version of Freothwulf retreating to the kingdom of Deira which originated as the civitas of the Parisi.
Those dates in the FH seem to indicate chronologically Freothwulf could not have been an adversary of Arthur. However, Urien fought against Theodoric and if Freothwulf preceded Theodoric then it is possible that Freothwulf was a contemporary of Arthur and the FH dating is incorrect. The FH does contain dates that may be questionable, such as Maelgwn’s death in the year 586.
According to the HB, Arthurs ninth battle occurred “in urbe legionis”. This may be a reference to York, which had been in the territory of the Brigantes according to Ptolemy and a Roman legionary base. Higham states that the Parisi were subordinate to the Brigantes. Frollo is said to have fled to a city so it may well be his duel with Arthur occurred on an island beyond the old York city walls.