In his letters to Louvois, Saint-Mars describes Dauger as a quiet man, giving no trouble, “disposed to the will of God and to the king”, compared to his other prisoners who were either always complaining, constantly trying to escape, or simply mad.
Dauger was not always isolated from the other prisoners. Wealthy and important ones usually had manservants; Fouquet for instance was served by a man called La Rivière. These servants, however, would become as much prisoners as their masters and it was thus difficult to find people willing to volunteer for such an occupation. Since La Rivière was often ill, Saint-Mars applied for permission for Dauger to act as servant for Fouquet. In 1675 Louvois gave permission for such an arrangement on condition that he was to serve Fouquet only while La Rivière was unavailable and that he was not to meet anyone else; for instance, if Fouquet and Lauzun were to meet, Dauger was not to be present.
It is an important point to note that the man in the mask served as a valet. Fouquet was never expected to be released; thus, meeting Dauger was no great matter, but Lauzun was expected to be set free eventually, and it would have been important not to have him spread rumours of Dauger’s existence. Historians have also argued that 17th-century protocol made it unthinkable that a man of royal blood would serve as a manservant – casting some doubt on those suggestions that Dauger was related to the king.
After Fouquet’s death in 1680, Saint-Mars discovered a secret hole between Fouquet and Lauzun’s cells. He was sure that they had communicated through this hole without detection by him or his guards and thus that Lauzun must have been made aware of Dauger’s existence. Louvois instructed Saint-Mars to move Lauzun to Fouquet’s cell and to tell him that Dauger and La Rivière had been released. In fact, they were held in another cell in another part of the prison, their presence there being highly secret.
The fate of the mysterious prisoner – and the extent of the precautions his jailers took – created significant interest and many legends. Many theories are in existence and several books have been written about the case. Some were presented after the existence of the letters was widely known. Later commentators have still presented their own theories, possibly based on embellished versions of the original tale.
Theories about his identity made at the time included that he was a Marshal of France; or the English Henry Cromwell, son of Oliver Cromwell; or François, Duke of Beaufort.
Voltaire claimed that the prisoner was a son of Anne of Austria and Cardinal Mazarin, and therefore an illegitimate half-brother of King Louis XIV. However, the authenticity of this claim is uncertain. Alexandre Dumas used this theory in his book The Vicomte de Bragelonne, but made the prisoner an identical twin of Louis XIV. This book has served as the basis – even if loosely adapted – for many film versions of the story.
Hugh Ross Williamson argues that the man in the iron mask was actually the father of Louis XIV. According to this theory, the “miraculous” birth of Louis XIV in 1638 would have come after Louis XIII had been estranged from his wife for 14 years. Furthermore, the king was old, weak, ill, and not expected to live much longer, and may have been impotent, which suggests that Louis XIII was not the father.
The suggestion is that the King’s minister, Cardinal Richelieu, had arranged for a substitute, probably an illegitimate son or grandson of Henry IV, to become intimate with the queen and father an heir. At the time, the heir presumptive was Louis XIII’s brother Gaston d’Orléans, who was also Richelieu’s enemy. If Gaston became King, Richelieu would quite likely have lost both his job as minister and his life, so it was in his interests to thwart Gaston’s ambitions. Louis XIII also hated Gaston and might have agreed to the scheme, and the queen would have had the same interest, as Gaston would have removed her from any influence.
In 1801, revolutionary legislator Pierre Roux-Fazillac stated that the tale of the masked prisoner was an amalgamation of the fates of two separate prisoners, Ercole Antonio Mattioli (see below) and an imprisoned valet named “Eustache D’auger”.
In his letter to Saint-Mars announcing the imminent arrival of the prisoner who would become the “man in the iron mask,” Louvois gave his name as “Eustache Dauger” and historians have found evidence that a Eustache Dauger was living at the time and was involved in shady and embarrassing events involving people in high places known as l’Affaire des Poisons. His full name was Eustache Dauger de Cavoye.