Constantine (, , fl. 520–523) was a 6th-century king of Dumnonia in sub-Roman Britain, who was remembered in later British tradition as a legendary King of Britain. The only contemporary information about him comes from Gildas, who castigated him for various sins, including the murder of two "royal youths" inside a church. The historical Constantine is also known from the genealogies of the Dumnonian kings, and possibly inspired the tradition of Saint Constantine, a king-turned-monk venerated in Southwest Britain and elsewhere. In the 12th century, Geoffrey of Monmouth included Constantine in his pseudohistorical chronicle Historia Regum Britanniae, adding details to Gildas' account and making Constantine the successor to King Arthur as King of Britain. Under Geoffrey's influence, Constantine appeared as Arthur's heir in later chronicles. Less commonly, he also appeared in that role in medieval Arthurian romances and prose works, and in some modern versions of the legend. History The 6th-century monk Gildas mentions Constantine in chapters 28 and 29 of work De Excidio et Conquestu Britanniae. Constantine is one of five Brittonic kings whom the author rebukes and compares to Biblical beasts. Gildas calls Constantine the "tyrannical whelp of the unclean lioness of Damnonia", a reference to the books of Daniel and Revelation, and apparently also a slur directed at his mother. This Damnonia is generally identified as the kingdom of Dumnonia in Southwestern Britain. Scholars such as Lloyd Laing and Leslie Alcock note the possibility that Gildas may have instead intended the territory of the Damnonii, a tribe in present-day Scotland mentioned by Ptolemy in the 2nd century, but others such as Thomas D. O'Sullivan consider this unlikely. Gildas says that despite swearing an oath against deceit and tyranny, Constantine disguised himself in an abbot's robes and attacked two "royal youths" praying before a church altar, killing them and their companions. Gildas is clear that Constantine's sins were manifold even before this, as he had committed "many adulteries" after casting off his lawfully wedded wife. Gildas encourages Constantine, whom he knows to still be alive at the time, to repent his sins lest he be damned. The murders may relate to a 6th-century cult in Brittany honoring the Saints Dredenau, two young princes killed by an ambitious uncle. Scholars generally identify Gildas' Constantine with the figure Custennin Gorneu or Custennin Corneu (Constantine of Cornwall) who appears in the genealogies of the kings of Dumnonia. Custennin is mentioned as