King Arthur is the figure at the heart of the Arthurian legends. Was King Arthur a historical figure or merely legend?  Modern scholars have assumed that there was some actual person at the heart of the legends, though not of course a king with a band of knights in shining armor.  The design of an order of the best knights in the world that figures in the major versions of the legend from Malory to Tennyson to T. H. White. Central to the myth is the downfall of Arthur's kingdom.

King Arthur was the son of Uther and defeated the barbarians in a dozen battles. Subsequently, he conquered a wide empire and eventually went to war with the Romans. He returned home on learning that his nephew Mordred had raised the standard of rebellion and taken Guinevere, the queen. After landing, his final battle took place.

The saga built up over the centuries. Malory's Le Morte D'Arthur would become what many considered the standard 'history' of Arthur. In this, we are told of Arthur's conception when Uther approached Igraine who was made, by Merlin's sorcery, to resemble her husband. The child was given to Ector to be raised in secret. After Uther's death there was no king ruling all England. Merlin had placed a sword in a stone, saying that whoever drew it out would be king. 
Arthur did so and Merlin had him crowned. This led to a rebellion be eleven rulers which Arthur put down. He married Guinevere whose father gave him the Round Table as a dowry; it became the place where his knights sat, to avoid quarrels over precedence. A magnificent reign followed, Arthur's court becoming the focus for many heroes. 

In the war against the Romans, Arthur defeated the Emperor Lucius and became emperor himself. However, his most illustrious knight, Lancelot, became enamoured of Guinevere. The Quest for the Holy Grial began and Lancelot's intrigue with the Queen came to light.

Lancelot fled and Guinevere was sentenced to death. Lancelot rescued her and took her to him realm. This led Arthur to crossing the channel and making war on his former knight. While away from Britain, he left Mordred in charge. Mordred rebelled and Arthur returned to quell him. This led to Arthur's last battle on Salisbury Plain, where he slew Mordred, but was himself gravely wounded. Arthur was then carried off in a barge, saying he was heading for the vale of Avalon. Some said he never died, but would one day return. However, his grave was supposedly discovered at Glastonbury in the reign of Henry II.