Where the Dark Stands Still by A. B. Poranek

Liska knows that magic is monstrous, and its practitioners are monsters. She has done everything possible to suppress her own magic, to disastrous consequences. Desperate to be free of it, Liska flees her small village and delves into the dangerous, demon infested spirit-wood to steal a mythical fern flower. If she plucks it, she can use its one wish to banish her powers. Everyone who has sought the fern flower has fallen prey to unknown horrors, so when Liska is caught by the demon warden of the wood - called the Leszy - a bargain seems better than death: one year of servitude in exchange for the fern flower’s wish.