Well, if the PCs are in possession of some valuable, powerful items (the "key") you have plenty of options:
- the "key" is to something bad, so good guys decide to take it from them to save the world, etyc...
- the "key" is good, so good guys try and take it from them to save the world
- the "key" is merely valuable, so bad guys try and take it from them to get rich
- the "key" is super-evil, the kind that even bad guys don't want, except fro the really, really bad -- and they want it.
- the "key" is cursed, and now so are the PCs, and only the long lost mulligan in the long lost mulligan-place can free them
In all of the above, I think NPCs make the best opposition. High level characters are generally far more dangerous and versatile that things with tons of hit dice. Plus, their motivations tend to be more fun.
Or, you can decide that whoever wants it, or wants to destroy it, or whatever is a big-old good dragon. A wyrm gold or silver, with all that implies, would likely give a 13th level party one hell of a chellnge.
