Interesting idea you have. It just needs to be tweaked a little bit....

Ideas:

Sylph stronghold is some giant tree structure and it is being invaded by either the empire or another beastmen tribe. Special doors are locked to advance to the next level of the structure and can only be opened by a sylph. You have to find a sylph that is either jailed up somehow or cornered by its attackers and use them to open the doors for you. Some doors will require multiple sylph to open.