Uther Pendragon (pen-dragon="son of the dragon") is the legendary father of King Arthur in the Arthurian legends, first mentioned by Geoffrey of Monmouth in his Historia Regum Britonniae. According to Geoffrey, he impregnated Igraine while magically disguised as her husband. The theme of illegitimate birth is repeated in Arthur's siring of Mordred.
The legendary figure of Uther may be based in part on the Romano-British warlord Ambrosius Aurelianus. Geoffrey lists Ambrosius as Uther's older brother and predecessor to the throne of Britain, along with a brother Constans, whom Vortigern had made his puppet king, all of whom were sons of a High-King called Constantine. This legendary High King may either be based on the historical Constantine III, a claimant to the Roman throne from 407-411, or king Constantine of Dumnonia who lived in the sixth century.
The main character of Aleister Crowley's Diary of a Drug Fiend claims to be descended from Uther Pendragon.