"Le Roi d'Ys" is an opera composed by Edgar Varèse with a libretto by Paul Corneille. It is based on the legend of Ys, a mythical city in Brittany that was said to be submerged by the sea. The story revolves around King Gradlon, who struggles to save his city and his daughter, Mélisande, from the consequences of a pact with the sea. The opera features themes of love, sacrifice, and the conflict between human desires and nature. It is notable for its rich orchestration and dramatic narrative, capturing the essence of the tragic fate of Ys and its inhabitants.