In the novel by Alexandre Dumas, 'The Man in the Iron Mask' is reputed to be what relation to the King Louis XIV?

The Man in the Iron Mask is the name given to an unidentified prisoner who was arrested in 1669 or 1670 and subsequently held in a number of French prisons, including the Bastille and the Fortress of Pignerol (modern Pinerolo, Italy). Recent research suggests that his name might have been "Eustache Dauger", but this still has not been completely proven. He was held in the custody of the same jailer, Bénigne Dauvergne de Saint-Mars, for a period of 34 years. He died on 19 November 1703 under the name "Marchioly", during the reign of Louis XIV of France (1643–1715). Since no one ever saw his face because it was hidden by a mask of black velvet cloth, the true identity of the prisoner remains a mystery even today.

Writer and philosopher Voltaire claimed in the second edition of his Questions sur l'Encyclopédie (published in 1771) that the prisoner wore a mask made of iron rather than of cloth, and that he was the older, illegitimate brother of Louis XIV. In the late 1840s, writer Alexandre Dumas elaborated on the story in the novel The Vicomte of Bragelonne: Ten Years Later, the final installment of his classic D'Artagnan saga: here the prisoner is forced to wear an iron mask and is Louis XIV's identical twin brother.