Kathleen Duey 
Silence and Stone 

Unicorns are now free, but other magical folk are still in hidingexcept for one faerie, Alida, who is a prisoner in Lord Dunravens castle. That is, until she escapes and a fierce struggle begins. Alida fights to free the faeries from the terrible promise they were forced to make to Lord Dunravenbut Lord Dunraven is desperate to continue denying his people the hope they derive from the existence of magical beings.