I'd poke around online, check some of the wikis. Failing that, have it be that the Fey decide to "help" the heroes in exchange for a favor, which'll allow you to shift enemy types.
That's what I'm doing with the Baba Yaga characters. She will give them aid or advice on how to return home if the complete some seemingly simple (but not really) task or if they give her a piece of themselves. My other idea was that an archfey would trick them into allowing him passage into the prime material plane.
Oooo, very clever. And yeah, that shift is a good way to open up potential other enemies.