Early introductions of arthur

The earliest appearances of King Arthur and Arthurian Legends are first known to be in the sixth century Welsh poem Y Goddodin:

This usage of Arthur's name suggests and earlier reference, or perhaps a historical meaning, but this the earliest documentation we have yet encountered out Arthurian legend. More Arthurian characters are encountered a few centuries later in The Mabinogion, another Welsh story in which we find Guinevere,Gawain, and the knights Kay and Bedivere.

Historia Regum Britanniae, written in 1139 by Geoffrey of Monmouth, is the earliest narrative continuously focused on Arthurian legend. Here, Arthur is introduced as the son of Uther Pendragon, and Merlin is introduced as the mentor of Arthur. All Arthurian stories beyond this point are based on these works.

Most Arthurian legends are based on stories and works of 5th and 6th century Celtic works. By the 10th and 11th centuries, these stories were known as far as Italy. French works became popular by the 12th century. Le Mort d' Arthur, a retelling of most of the Arthurian works, written in 1485 by Sir Thomas Malory is one of the most famous and popular of the tales. (2)

Map of the arthurian land.(3)

Sources

(1) http://www.centralparknyusa.com/topic/The_Gododdin
(2) http://sangha.net/messengers/king-arthur.htm
(3) http://www.kingarthursknights.com/etc/map.asp