Deruvian (Medieval Latin: Deruvianus), also known by several other names including Damian, was a possibly legendary 2nd-century bishop and saint, said to have been sent by the pope to answer King Lucius's request for baptism and conversion to Christianity.

His feast day does not appear in any medieval Welsh calendar of the saints and is not presently observed by the Anglican, Catholic, or Orthodox churches in Wales.

Deruvian's name is also cited as "Duvian" (Duvianus)[1][2] or "Dwywan"[3] and, owing to scribal error, also appears in modern saints' lists as "Damian" (Damianus).

[3] The legendary accounts of King Lucius of Britain's baptism during the late-2nd-century pontificate of Eleutherius are documented at least as far back as the "Felician Catalog", an early 6th-century edition of The Book of Popes which added more details to the terser entries of earlier recensions.

Around 1203, Gerald of Wales's composed his work On Invectives, which claims to preserve verbatim a letter to Pope Honorius II from the convent of St David's.

[23] Shortly after, Geoffrey of Monmouth's pseudohistorical History of the Kings of Britain also described "Duvian" as the companion of "Fagan", providing many additional but suspect details.

[1][2] Baring-Gould, Rees and Mullins are widely dismissive of the legends surrounding Lucius but offer that Deruvian and his companion may have been genuine local saints whose names were preserved in the area around Llandaff and then—as else nothing remained known of them—were mixed up with the separate stories surrounding Lucius[22][24][23] Bartrum, however, notes the lack of earlier sources and posits that one must suppose such dedications followed the popularization of Geoffrey's story.

[29] Roberts notes the similarity with Dyfnan, son of the invading Irishman Brychan of Brycheiniog, and finds it unlikely to have been associated with a Dyfan, "because the place was always called Llandyfân with the accent on the last syllable", appearing in earlier records as Llanduvaen.

[26][36] He and St Fagan took on renewed importance during the English Reformation: at his martyrdom in 1604, the Blessed John Sugar asked his Protestant accusers who had evangelized the country; receiving no answer, he listed Eleutherius, "Damianus", and "Fugatius" as evidence for the early date of British Catholicism.