Le Lai De Lanval by Marie de France

Set in the time of King Arthur, the story follows a noble knight who, despite his valor and loyalty, is overlooked and neglected by the court. In his solitude, he encounters a mysterious and enchanting fairy lady who offers him her love and wealth, with the condition that he must never reveal her existence. As their secret romance blossoms, the knight's loyalty is tested when he is falsely accused by the queen of rejecting her advances. To prove his innocence and honor, he must rely on the magical intervention of his beloved, leading to a dramatic and enchanting resolution.

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