Oh I don’t know. I just thought it’d be a cool idea and so I posted it. If I was currently DMing then I would figure that part out. Maybe the “keys” reveal details about the BBEG like they’re all of dwarven make or a certain script. Maybe one key could only have been made by some famous smith in a certain town.
This was just part of an idea I thought I’d post since it sounded fun. I replied to the other commenter with similar questions but I’m not currently DMing otherwise I would figure that out.