Uther Pendragon (pen-dragon:"head of dragons" in Breton) is the father of King Arthur in Arthurian legend. His name seems to be derived from or have the same root as Arthur. The nickname Pendragon comes from a comet in the shape of a dragon that Uther "sees" and from which he draws inspiration to create two dragon-standards. Another tradition says that Uther carried in his saddle the heads of two dragons, one white and one red, who lived underground and who were awakened by the weight of the tower that King Vortigern had built above them. Coming out of the ground, the dragons killed each other.
Uther is first mentioned by Geoffrey of Monmouth in his Historia regum Britanniae (History of the Kings of Britain). According to Geoffroy, he impregnates Ygraine by taking on the semblance of her husband Gorlois, Duke of Cornwall, through Merlin's magic. From this pregnancy will be born Arthur. The theme of illegitimate birth is repeated in later Arthurian romances with Mordred, sired by Arthur, and Gilead, son of Lancelot.
Geoffroy makes Uther the youngest brother of Ambrosius Aurelianus (historical figure) and his successor to the throne of Logres, both being younger brothers of Constans, whom Vortigern had made his puppet king before his death, all three being sons of a king named Constantine of Damnonia. This legendary king is perhaps partly the result of a confusion with King Constantine III, having really existed, claiming the imperial throne of Rome between 407 and 411, and King Constantine of Damnonia who lived in the 6th century and perhaps also to be with a third Constantine who appears in the Welsh Genealogies.
The Welsh text mentions another son of Uther, Madoc, father of Arthur's nephew, Eliwlod.
Geoffroy also attributes to Uther a daughter, Anna (called elsewhere Morgause and sister of Morgane and half-sister of Arthur), sister of Ygraine. Anna marries King Lot and becomes the mother of Gawain (Gawain in English) and Mordred. But in later novels Gauvain's mother is usually Eleine, the previously married daughter of Ygraine and Gorlois. Similarly, elsewhere in his work, Geoffroy claims that King Lot had married Aurelius' sister during his reign.
In the Welsh Genealogies, Anna appears as the mother of Howel of Brittany (whom Geoffroy also makes a nephew of Arthur). But in these genealogies this Anna appears unrelated to Uther Pendragon. Geoffroy also insists a lot on a line of kings of Brittany of British origin that Uther "sees" in the vision he has of a comet personified by a daughter of his descent; the first of these Breton kings being Howel. Thus, it seems that Geoffroy's "Anna" appeared in all the sources he used as being Howel's mother, not Gauvain's. Later narrators departing from Geoffroy's account remain bewildered by these contradictions and generally attempt to resolve them by making Howel's kingdom an Arthurian fact.
There is another tale with the character of Uther Pendragon as a backdrop, Wolfram of Eschenbach's Parzifal. A certain Mazadan goes with a fairy, named Terdelaschoye, to the country of Feimurgan. This looks like some distorted original source recounting the alliance of Mazadan with the Fairy Morgana, fairy of the Land of Joye. Mazadan becomes the father of two sons, Lazaliez and Brickus. The latter became the father of "Utepandragun", himself the father of Arthur, while the eldest, Lazaliez, became the father of Gandin d'Anjou father of Gahmuret, father of Parzifal/Perceval. Both Uther Pendragon and Arthur appear here as offshoots of a minor branch of an imaginary 5th/6th century House of Anjou.
In the Prose Lancelot, Uther Pendragon claims to have been born in Bourges. He gathers an army to go to Brittany to fight King Claudas de Bourges, a situation not unlike that which took place in the 5th century with the British leader Riothamus who went to Brittany to fight the raiders who were rampant in Bourges.
In Robert de Boron's Merlin, Uther Pendragon kills the Saxon Hengist (the name Hengist is equivalent to Angis or Augis) as Hengest intrudes into the British camp with the intention of assassinating him. It is for Uther Pendragon that the Round Table was created by Merlin.
According to a theory by David Sims and Mick Baker, Uther Pendragon could have been the Welsh king Einion the Impetuous, due to a series of coincidences:The Impetuous, calling himself "Yrth" could have given "Yrthr" ("Uther ), and as head of the royal family of Gwynedd, Einion bore the title of Pen Draig "Chief Dragon". The name "Yrthr Pen Draig" is reminiscent of "Uther Pendragon". In addition, other coincidences relate to his son, Owain Dents-Blanches who could have been King Arthur. These are of course only conjectures given the few documents that we have from this period.