The trolls will ask you a lot of money for their work.

     Although their king rarely works because his skin allows him to pretend he's not there at all. It can hide itself very well into any landscape. This stratagem should bear fruit, in theory. But their opponents... OH WATCH OUT! It is flying above you! It will try to convert you, but it will first allow you to ask for one wish. I have to warn you : it has to be a passive wish.