Crown of Midnight by Sarah J Maas

Eighteen-year-old Celaena Sardothien survived a year as a slave in the brutal prison camp at Endovier and battled criminals and evil monsters to win the title of King's Champion. Now she must work as the royal assassin to Adarlan's evil king for four years to win her freedom.

This book contains; alcohol use, animal cruelty, death, deception, inexplicit sexual activities, nudity,  mild/infrequent profanity, and violence.