Bitterthorn by Kat Dunn

Blumwald is a town overshadowed by an ancient curse: in a sinister castle in the depths of the wild wood lives a monstrous Witch. Once a generation, she comes to claim a companion to return with her – never to be seen again. Now that time is drawing near once more…
Mina, daughter of the duke, is grieving and lonely. She has lost all hope of any future for herself in Blumwald. So when the Witch demands her next companion, Mina offers herself up – though she has no idea what fate awaits her. Stranded with her darkly alluring captor, the mystery of what happened to the previous companions draws Mina into the heart of a terrifying secret that could save her life, or end it.