Ever read Alexander Dumas' "The Man in the Iron Mask"?. Seen either version of the movie with Richard Chamberlain or Leonardo DeCaprio?
Per "Mysteries of Templar Treasue & the Holy Grail- The Secrets of Rennes-le-Chateau", by Lionel & Patricia Fanthorpe. Weiser Books, 1992 page 126:
The man in the iron mask could have been a real figure in history. What could this prisoner have to do with Rennes-le-Chateau? Rules of imprisonment: No one except St. Mars, King Louis XIV's trusted agent and prison governor, was to speak to the masked man, or to listen to anything he might try to say. If they did, he was to be executed immediately. The masked man was treated with great respect and courtesy as if he was of royal blood, or at least of very high rank. This is a very dangerous prisoner to someone. But did he have a secret that somebody wanted? It is suggested that Minister Nicolas Fouquet was the person with the secret. ( Fouquet is a character in Dumas' book). Was the secret related to ultimate weapons, the secret of transmutations or the Elixir of Life? If Fouqhet was this man, then the secret he withheld from Louis may have been linked to the secret of Rennes-le-Chateau. Others believe the secrets of the Cathars, the troubadours, the Templars, the Alchemists and the Rosicrucians came into the hands of the Freemasons.
See this page for more discussion of the Man In the Iron Mask.