It started in 1698, when a prisoner using the pseudonym Eustache Dauger (which on paper looks heroically similar to "Mustache Danger") was transported to the Bastille, the 17th century French equivalent of a maximum security prison. He had already spent between two and three decades (old-timey records are, at best, imprecise) rotting in various jails across the country. According to legend, he showed up already locked in the iron mask (which looked like an Iron Man Mark I helmet) and was immediately tossed in a cell, forbidden to speak to anyone except to ask for food, water, or a pot to make poopies.
"OK, I filled the pot. Now can I have my stale bread and water?"
And ... that's all we know. Nobody discussed who the hell he was or what he had done or why he needed to be dressed like a very lazy knight. He was listed in the Bastille records as "Prisoner 64389000," which, in addition to being difficult to fit into a song, is a completely sterile piece of information. He was forbidden to ever show his face to anyone, and some prisoners claimed he had two armed guards with him at all times should he ever try to take the mask off. In that event, we assume they would aim for his chest, or shove their musket barrels into his iron eyeholes.