The Man in the Iron Mask was "an unidentified prisoner of state during the reign of Louis XIV of France." According to Wikipedia, he was arrested in 1669 under the name "Eustache Dauger" and "held for 34 years in the custody of Bénigne Dauvergne de Saint-Mars in four successive French prisons, including the Bastille." He died there in 1703, and his burial certificate bore the name "Marchioly."
Surviving documents describe him as "only a valet," jailed for "what he was employed to do" and for "what he knew." The famous mask was mostly myth: "Legend has it that no one ever saw his face, as it was hidden by a mask of black velvet cloth, later misreported by Voltaire as an iron mask." Records show he had to cover his face only when traveling between prisons after 1687 or when going to prayers within the Bastille.
Voltaire claimed the prisoner was an illegitimate older brother of the king; Alexandre Dumas later made him Louis XIV's identical twin. In all, "more than 50 candidates, real and hypothetical, have been proposed," and his true identity "remains a mystery."
