Uther Pandragon is a legendary king of Britain and the father of King Arthur.

A few minor references to Uther appear in Old Welsh poems, but his biography was first written down by Geoffrey of Monmouth in his Historia Regum Britanniae (History of the Kings of Britain), and Geoffrey's account of the character was used in most later versions.

Uther Pendragon, throughout Arthurian literature, is described as a strong king and a defender of the people.

Uther Pendragon, with Merlin's help, tricks Igraine the wife of his enemy, Gorlois. The Lady Igraine, sees Uther Pendragon, and believes he is her husband, Gorlois. While her husband is away, Uther sleeps with Igraine at Tintagel. Igraine conceives Arthur, "the once and future king."

This act of conception occurs the very night Uther's troops dispatch Gorlois, Lord of Cornwall. This theme of illegitimate conception is repeated in King Arthur's siring of Mordred on his own sister Morgause in the later prose romances. It is Mordred who will eventually mortally wound King Arthur in the Battle of Camlann.
 

Goeffrey of Monmouth
Uther's epithet Pendragon means "Chief Dragon" in a figurative sense, referring to his status as "foremost leader" or "chief of warriors". The name was possibly misinterpreted by Geoffrey of Monmouth in the Historia to mean "dragon's head". According to Geoffrey of Monmouth and works based on his version, Uther acquires the epithet when he witnesses a portentous dragon-shaped comet, which inspires him to use dragons on his standards.

Sir Lancelot

According to the Lancelot-Grail or Vulgate cycle, it was Uther's older brother (elsewhere called Ambrosius Aurelianus) who sees the comet and receives the name "Pendragon"; Uther takes his epithet in his honour after his brother's death. Of course, the other possibility is that Geoffrey of Monmouth knew the story of the comet, and tried to find a way of translating the name Pendragon to make sense of it.

Early Welsh Poetry
Uther Pendragon is known from earlier Welsh tradition, where he is associated with King Arthur and, in some cases, even appears as his father. He is mentioned in the 10th century Arthurian poem Pa gur yv y porthaur ("What man is the gatekeeper?") and is memorialized with "The Death-song of Uther Pen" from the Book of Taliesin. The latter includes a reference to King Arthur, so the marginal addition of "dragon" to Uther's name is probably justified. "The Colloquy of Arthur and the Eagle," a poem contemporary with but independent of Geoffrey of Monmouth, mentions another son of Uther named Madoc, the father of King Arthur's nephew Eliwlod. The Welsh Triads name Uther as the creator of one of the Three Great Enchantments of the Island of Britain, which he taught to the wizard Menw.

History of the Kings of Britain
Uther Pendragon is best known from Geoffrey of Monmouth's History of the Kings of Britain (1136) where he is the youngest son of King of Britannia Constantine II. His eldest brother Constans succeeds to the throne on their father's death, but is murdered at the instigation of his adviser Vortigern, who seizes the throne. Uther Pendragon and his other brother Aurelius Ambrosius, still children, flee to Brittany. After Vortigern's alliance with the Saxons under Hengist goes disastrously wrong, Aurelius and Uther Pendragon, now adults, return. Aurelius burns Vortigern in his castle and becomes king.

With Aurelius on the throne, Uther Pendragon leads his brother's army to Ireland to help Merlin bring the stones of Stonehenge from there to Britain. Later, while Aurelius is ill, Uther Pendragon leads his army against Vortigern's son Paschent and his Saxon allies.

On the way to the battle, he sees a comet in the shape of a dragon, which Merlin interprets as presaging Aurelius's death and Uther's glorious future. Uther wins the battle and takes the epithet "Pendragon", and returns to find that Aurelius has been poisoned by an assassin.

Uther Pendragon becomes king and orders the construction of two gold dragons, one of which he uses as his standard. He secures Britain's frontiers and quells Saxon uprisings with the aid of his retainers, one of which is Gorlois, Duke of Cornwall. At a banquet celebrating their victories, Uther becomes obsessively enamoured of Gorlois' wife, Igerna (Igraine), and a war ensues between Uther Pendragon and his vassal.

Gorlois sends Igraine to the impregnable castle of Tintagel for protection, while he himself is besieged by Uther in another town.

King Uther Pendragon consults with Merlin, who uses his magic to transform the king into the likeness of Gorlois and thus gain access to Igraine at Tintagel. He spends the night with her, and they conceive a son, Arthur; but the next morning it is discovered that Gorlois had been killed. Uther marries Igraine, and they have another child, a daughter called Anna. She later marries King Lot and becomes the mother of Gawain and Mordred (in later romances she is called Morgause, and is usually Igraine's daughter by her previous marriage).

Uther Pendragon later falls ill, but when the wars against the Saxons go badly he insists on leading his army himself, propped up on his horse. He defeats Hengist's son Octa at Verulamium (St Albans), despite the Saxons calling him the "Half-Dead King." However, the Saxons soon contrive his death by poisoning a spring he drinks from near Verulamium.

Geoffrey of Monmouth based some members of Uther's family on historical figures. Constantine is based on the historical usurper Constantine III, a claimant to the Roman throne from 407–411; Constans is based on his son. Aurelius Ambrosius is based on the legendary Welsh figure Ambrosius Aurelianus, though his connection to Constantine and Constans is an invention. It is possible, however, that Uther Pendragon existed as an historical figure and an important character of Britain's mythical history.

Medieval Literature
In Robert de Boron's Merlin, Uther Pendragon personally kills Hengest after an assassination attempt by the Saxon leader, and Merlin creates the Round Table for him. In Prose Lancelot, Uther Pendragon claims to have been born in Bourges. He takes an army to Brittany to fight against King Claudas of Bourges, a situation resembling that of the historical ruler, Riothamus, who went to Brittany to fight ravagers based in Bourges.

Modern Literature
Uther Pendragon remains a widely used character in modern Arthurian literature. In T.H. White's The Once and Future King, Uther the Conqueror is the Norman King of England from 1066 to 1216. Mary Stewart's first two books in her Arthurian saga, The Crystal Cave, The Hollow Hills, feature Uther Pendragon. Notably, he is Merlin's uncle, since in this version the latter is his brother Ambrosius' illegitimate son.

In Bernard Cornwell's The Warlord Chronicles, Uther is the King of Dumnonia as well as the High King of Britain. In Jack Whyte's The Camulod Chronicles, Uther is King of the Pendragon, the Celtic people of South Cambria, cousin to Caius Merlyn Britannicus and Ambrose Ambrosianus Britannicus. In contrast to traditional versions, Stephen R. Lawhead's Pendragon Cycle makes Uther's brother Aurelius, whose widow (Ygerna) he marries, Arthur's true father.

In Marion Zimmer Bradley's Mists of Avalon, Uther Pendragon is the nephew of Aurelianus instead of his brother; while Aurelianus is the son of a Roman Emperor, Uther has no Roman blood. In Valerio Massimo Manfredi's The Last Legion, Uther is himself a Roman Emperor - the last, Romulus Augustus. While the real Romulus Augustus disappeared from history after being deposed by Goths, in the novel he escapes to Britain, where he adopts the name Pendragon and eventually sires Arthur.