The Man in the Iron Mask existed. He stayed in the mask for 34 years leading to much speculation about his identity and why the king of France would want his face hidden and why he was kept alive instead of executed
“The Man in the Iron Mask (French: L’Homme au Masque de Fer) is a name given to a prisoner arrested as Eustache Dauger in 1669 or 1670, and held in a number of jails, including the Bastille and the Fortress of Pignerol (today Pinerolo). 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 of Marchioly, during the reign of Louis XIV of France (1643–1715). The possible identity of this man has been thoroughly discussed and has been the subject of many books, because no one ever saw his face, which was hidden by a mask of black velvet cloth.
In the second edition of his Questions sur l’Encyclopédie (French for “”Questions on the Encyclopedia””), published in 1771, the writer and philosopher Voltaire claimed that the prisoner wore an iron mask and was the older, illegitimate brother of Louis XIV. In the late 1840s, the writer Alexandre Dumas elaborated on the theme in the final installment of his Three Musketeers saga: here the prisoner is forced to wear an iron mask and is Louis XIV’s twin brother.
What facts are known about this prisoner are based mainly on correspondence between his jailer and his superiors in Paris.”