Mordred, the Judas of the Arthurian cycle--whom some poets represent as the illegitimate and incestuous son of Arthur, while others merely make him a nephew of the king--rebels against Arthur, who engages in his last battle, near the Castle of Tintagel, where he was born.
In this encounter all are slain on both sides, and Arthur, having finally killed the traitor Mordred, after receiving from him a grievous wound, finds no one near to help or sustain him save Sir Bedevere.

Knowing his wonderful blade Excalibure must return to its donor ere he departs, Arthur thrice orders his henchman to cast it into the mere.

Twice Sir Bedevere hides the sword instead of obeying, but the third time, having exactly carried out the royal orders, he reports having seen a hand rise out of the Lake, catch and brandish Excalibure, and vanish beneath the waters with it! Arthur is next carried by Sir Bedevere down to the water's edge, where a mysterious barge receives the almost dying king.

In this barge are three black-veiled queens,--the king's step-sisters,--and, when Arthur's head has been tenderly laid in the lap of Morgana the Fay, he announces he is about to sail off to the Isle of Avalon "to be healed of his wound." Although the Isle of Avalon was evidently a poetical mediaeval version of the "bourne whence no man returns," people long watched for Arthur's home-coming, for he was a very real personage to readers of epics and romances in the Middle Ages.
Guinevere--her sin having been discovered by her hitherto fabulously blind husband--took refuge in a nunnery at Almesbury, where she received a farewell visit from Arthur and an assurance of his forgiveness, before he rode into his last fight.
As for Launcelot, he, too, devoted his last days to penance and prayer in a monastery.

There he remained until warned in a vision that Guinevere was dead.