Arthur is a central figure in the legends making up the so-called Matter of Britain. The legendary Arthur developed as a figure of international interest largely through the popularity of Geoffrey of Monmouth's fanciful and imaginative 12th-century Historia Regum Britanniae (History of the Kings of Britain). In some Welsh and Breton tales and poems that date from before this work, Arthur appears either as a great warrior defending Britain from human and supernatural enemies or as a magical figure of folklore, sometimes associated with the Welsh Otherworld, Annwn. How much of Geoffrey's Historia (completed in 1138) was adapted from such earlier sources, rather than invented by Geoffrey himself, is unknown.
Although the themes, events and characters of the Arthurian legend varied widely from text to text, and there is no one canonical version, Geoffrey's version of events often served as the starting point for later stories. Geoffrey depicted Arthur as a king of Britain who defeated the Saxons and established an empire over Britain, Ireland, Iceland, Norway and Gaul. Many elements and incidents that are now an integral part of the Arthurian story appear in Geoffrey's Historia, including Arthur's father Uther Pendragon, the wizard Merlin, Arthur's wife Guinevere, the sword Excalibur, Arthur's conception at Tintagel, his final battle against Mordred at Camlann, and final rest in Avalon. The 12th-century French writer Chrétien de Troyes, who added Lancelot and the Holy Grail to the story, began the genre of Arthurian romance that became a significant strand of medieval literature. In these French stories, the narrative focus often shifts from King Arthur himself to other characters, such as various Knights of the Round Table. Arthurian literature thrived during the Middle Ages but waned in the centuries that followed until it experienced a major resurgence in the 19th century. In the 21st century, the legend lives on, not only in literature but also in adaptations for theatre, film, television, comics and other media.
reference : Wikipedia