Geoffrey of Monmouth
Writing in Latin, Geoffrey adapted the Welsh legend of Myrddin Wyllt, altered his name to Merlin and made the character a central figure in his three books: Prophetiae Merlini, Historiae Regum Britanniae and Vita Merlini.
The first of these, written before 1135, was the first work about the mythical wildman and prophet written in a language other than Welsh. According to Geoffrey, Merlin was born in Carmarthen.
Written around 1136, Geoffrey's second book, Historiae Regum Britanniae, brought together tales of Merlin and Arthur. In the book, Merlin appears in the tales of kings Vortigern, Aurelius Ambrosius and Uther Pendragon. When Vortigern invites the Saxons to fight as mercenaries, it leads Britain to a state of seige which continued under his successors Aurelius Ambrosius and Uther Pendragon.
It is the latter character who, disguised by the wizard Merlin, enters Tintagel Castle in Cornwall to sleep with Igraine (Eigyr in Welsh legend). Igraine gives birth to Arthur at Tintagel, and later marries Pendragon.
According to Geoffrey, it is Arthur who quells the Saxon threat during his reign. He goes on to conquer much of northern Europe until his nephew Modred seizes the throne. Arthur returns and kills Modred, though is mortally wounded. He is taken to Avalon, where he hands his kingdom to his cousin Constantine.
Much of the romanticism of the Arthurian legend, including the tales of Camelot, Avalon, the Holy Grail, and Lancelot, Gawain and Galahad, came later, expanded upon by folklore and successive writers.
However, the story of Arthur's sword, Excalibur, may have had its roots in Welsh literature. While it was not said to have possessed magical properties, Geoffrey of Monmouth mentions a weapon, Caliburn, wielded by Arthur, which would "carve their souls from out them with their blood."
Although more storytelling than pure history, Geoffrey's writings became hugely popular. They were influential sources for medieval literature, and for several centuries his ideas were incorporated into scores of ballads and works of literature.
His writing helped give British consciousness a heroic figure to be proud of, and gave them hope in times of struggle against the Saxons.
It is fair to say that, were it not for Geoffrey of Monmouth, the tales of Arthur and his knights may not have endured as it has; his tales greatly influenced the later well-known works by Sir Thomas Malory and Alfred Lord Tennyson.