The Castle of Thorns

To end the murders, she must live with the beast of the forest. After surviving years with a debilitating illness, Gisela must prove she is more than her ailment. She discovers her father, King Werner of Tursch, has been growing desperate for the herbs that are key to her health. So much so, he's willing to cross paths with a deadly legend of Todesfall Forest to retrieve the remedy. Knorren is the demon of the forest, one who slaughters anyone who trespasses into his land.
When King Werner steps into the demon's territory, desperately pleading for safe passage for the sake of his daughter's health, Knorren toys with the idea. However, not without a cost. King Werner must deliver his beloved Gisela to the demon or suffer dire consequences. With unrest spreading through the kingdom, and its people growing tired of a king who won't put an end to the demon of Todesfall Forest, Gisela must make a choice.
To become Knorren's prisoner forever, or risk the lives of her beloved people. Perfect for fans of Holly Black and Sarah J. Maas. This is a dark retelling you won't want to miss out on!