He had a brother living in Arabia, to whom he related the miracles he had beheld, and who was thus called to the path of salvation, came to Jerusalem, was baptised by the name of Cecilius, and was charged, together with Ctesiphon, to assist the deacons in the newly-formed Christian community.
Ctesiphon accompanied the Apostle St.James the Greater into Spain, and also returned with him.

After a time, he was again sent into Spain by the Apostles, and carried there the body of St.James, who had been martyred at Jerusalem.

He was made a bishop, and resided chiefly in a sort of island or peninsula at no great distance from France, which he also visited, and where he made some disciples.

The name of the place where he lived was rather like Vergui, and it was afterwards laid waste by an inundation.

I do not remember that Ctesiphon was ever martyred.
He wrote several books containing details concerning the Passion of Christ; but there have been some books falsely attributed to him, and others, which were really from his pen, ascribed to different writers.
Rome has since rejected these books, the greatest part of which were apocryphal, but which nevertheless did contain some few things really from his pen.