Some quick context.
The players got themselves arrested and thrown into jail in order to be at a certain location at a certain time. When the time was right they broke out and started moving their way through the keep in which they were kept. While escaping the jail they came across another prisoner who was obviously tortured and in bad shape, they free him and bring him along as they escape and do their original mission.
Along the way the players suss out that the prisoner can’t remember his name or anything before he was bound and chained and thrown in the cell and the manacles around his wrist that they couldn’t get off are actually magical in nature and might have something to do with his memory loss.
They eventually come across an NPC who is utterly shocked and full of dread when they realize that this prisoner is free and warn the players to not free him of his manacles, but this npc is also a member of an enemy faction and they players don’t really have a reason to trust anything he says.
The question I have is what is the mystery behind the mysterious prisoner with no name and no past?
I have a few ideas but I like hearing ideas from people.
One I have is kinda cliche. Hes a powerful mage blah blah and he eventually becomes the Big Bad once they players find a way to free him of his magical bindings.
One, more specific to the immediate situation is that he is the host to a powerful demon and once the magical bindings are removed the demon will be free to do his dark business.