A Throne of Blood and Ice by T.A. Lawrence

Silly servant girl, only princesses get happy endings.
Blaise wasn’t supposed to be the villain. She was supposed to charm the prince, the male she’s loved for years. That was all the potion was supposed to do—give her another face, and only for a few hours. All so she could prove the prince loved her. Except Blaise got more than she bargained for, and instead of a beauty potion, now she’s infected with a magical parasite who takes control of her body every full moon. A parasite who would rather take control permanently. When Blaise is kidnapped from her prison cell, she wakes in a dungeon only to find herself the object of the Queen of Mystral’s magical experiments. The queen wants the parasite. The parasite wants Blaise. And Blaise wants… Nox, the brainy, sarcastic fae tasked with extracting the parasite from Blaise’s body. Nox is different from any fae she’s met. For one, he can actually lie. Then there was that time she sliced her finger and he couldn’t stop staring at the drop of blood. The queen definitely has something over him, but Blaise can’t figure out what. She just has a feeling she won’t be making it out of these dungeons alive.