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In Thomas Mallory's Le Morte d'Arthur, there are several motifs, including the wise old man (Merlin), magic (Merlin and the Lady of the Lake), faithful followers (the Knights of the Round Table), tests of faithful followers (any of the many exploits of the Knights), dreams (e.g., Arthur's dream of truce, and the dream of the King with the Hundred Knights), and betrayal.
An in-depth example of betrayal is the relationship between Sir Lancelot and Queen Guinevere (Arthur's wife). Lancelot is one of Arthur's trusted knights, who traveled from France and joined the other knights that serve Arthur. Lancelot becomes Guinevere's "courtly lover."
He is practicing a medieval convention in which a knight chastely loves and honors a lady without regard to her marital status.
At first, Lancelot simply shows a high regard for Guinevere: it is a completely innocent relationship. However, it eventually turns into a love affair—adultery. Their actions will eventually lead to the downfall of Camelot and Arthur's Knights of the Round Table, and the King's death (among many others).
Lancelot promises to put his love for Guinevere behind him, however, the text says that he spent too much time thinking of Guinevere and not enough time considering his responsibility to God. Again he gives in to his feelings for the Queen:
Then, as the book saith, Sir Launcelot began to resort
unto Queen Guenever again, and forgat the promise and
the perfection that he made in the quest.
...ever his thoughts were privily on the queen, and so
they loved together more hotter than they did to-forehand…
Lancelot fights and kills many knights when he is caught with Guinevere. He tries to take Guinevere, but she will not leave. The Queen is sentenced to be burned at the stake, but Lancelot saves her. Ultimately, she returns to Arthur, and Lancelot leaves for France. Gawain urges Arthur to go to France to pursue Lancelot— which he does—but when Mordred threatens Arthur’s throne in England, Arthur returns and war ensues with Arthur's son.
Arthur is killed; Guinevere enters a convent. Lancelot searches until he finds her. Guinevere is overwrought upon seeing him, still very much in love, but she is deeply aware of what they have done. She explains to the other women:
Through this man and me hath all this
war been wrought, and the death of the most noblest
knights of the world...
She asks Lancelot to promise to leave and never return. By doing penance, she believes she may one day enter heaven despite what they have done, so she says she will stay and try to atone for her sins for the rest of her life.
Lancelot asks for a final kiss, but Guinevere refuses. Lancelot leaves, weeping. He seeks out a hermitage (holy place of recluse from the world) and there meets the Bishop of Canterbury. He asks the Bishop to shrieve (forgive) him of his sins and asks if he can become his "brother"...in other words, become a hermit himself.
Then the Bishop said: I will gladly; and there
he put an habit upon Sir Launcelot, and there he served God
day and night with prayers and fastings.
Lancelot and Guinevere's betrayal cost the lives of countless men, and brought about the destruction of Arthur, his Knights of the Round Table, and the mythical Camelot.