I like it! How should they be selected?? Through a kingdom/higher power or maybe a person that kinda knows all of them? 
This is just a thought but maybe the powerful/dangerous thing was something that was protected by either the wood elves, dwarfs or mages for like centuries until it was stolen. Knowing the destruction it's loss could bring, the village/society could choose to send out search parties for it as well as someone (one of our characters) to get help from the other villages/societies since it's a threat to them all. 

And I could tie Clarcia in by over hearing the word about the danger. I imagine she'd want to track them down (possibly looking for a reward for herself as much as wanting to stop the bad stuff xD) so she'd end up helping the group as well since she's good at tracking down people.