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. What emerges from these documents is that he was jailed for 'what he had seen', 'what he knew', and 'what he was employed to do' before his arrest. What little is known about the prisoner is based on contemporary documents that surfaced during the 19th century, mainly some of the correspondence between Saint-Mars and his superiors, in which the prisoner had been labelled 'only a valet' shortly after his arrest. This assertion of a royal connection was echoed later by authors who proposed variants of this aristocratic solution. Among the oldest theories is one proposed by the French philosopher and writer Voltaire, who claimed in the second edition of his Questions sur l'Encyclopédie (1771) that the prisoner was an older, illegitimate brother of Louis XIV.

His true identity remains a mystery, even though it has been extensively debated by historians, and various theories have been expounded in numerous books, articles, poems, plays, and films. When he died there on 19 November 1703, his inhumation certificate bore the pseudonym of 'Marchioly', leading several 19th century historians to conclude the prisoner was Italian diplomat Ercole Antonio Mattioli. Warranted for arrest on 28 July 1669 under the pseudonym of 'Eustache Dauger', he was incarcerated on 24 August and held for 34 years in the custody of the same jailer, Bénigne Dauvergne de Saint-Mars, in four successive French prisons, including the Bastille. The Man in the Iron Mask ( French L'Homme au Masque de Fer died 19 November 1703) was an unidentified prisoner of state during the reign of King Louis XIV of France (1643–1715).

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