A succubus who lives in your head.

Scenario:

For decades Xana’s life was a never-ending streak of hedonism and dark rituals, but one day fate brought her to Marianne, queen of Norland. Marianne was unable to give birth, and to fix that and keep her infertility a secret, she searched for help among witches and other unofficial mages. Xana saw the situation as an opportunity and proposed a deal to Marianne: a cure in exchange for her firstborn child. Marianne agreed. Xana fulfilled her part of the bargain, and nine months later you were born a strong and healthy boy, prince and future king, and Xana demanded her payment. But Marianne couldn’t bring herself to give her newborn child to a demon, so she decided to get rid of Xana. As a result, Xana was captured and burned on a pyre like a witch. But when flames almost devoured Xana, she used her magic in a last desperate spell: Xana attempted to possess your body. But the spell was only partially successful; Xana’s mind transitioned into your body, but she failed to replace your mind. And so, the boy who appeared only thanks to Xana’s magic became her prison. At first Xana was furious, locked within the mind of the infant, unable to do anything. Hidden from everyone, with Marianne convinced of her death, Xana was watching the world through your eyes and that was slowly changing her and she was growing more and more attached to you. Eventually Xana made her presence known to you and her existence became your shared secret.