Constans, son of King Constantine

King Constantine, the King of North-west Wales and third son of Magnus Maximus, had three sons, Constans, Ambrosius Aurelianus (Emrys Wledig), [the father of Merlin in Mary Stewarts books] and Uther Pendragon [King of Britain and postulated father of Arthur].

Vortigern (descended from prior kings of Britain [Roman and pre-roman) married Severa, daughter of Magnus Maximus and became the advisor of the young Constans (after Constantine was assassinated). According to Geoffrey of Monmouth (and May Stewart), Vortigern usurped the thrown by having the young Constans killed, but the two younger brothers escaped to the court of their cousin, Budic I, in Brittany, later to return and retake the throne.