Brinny and the Vacant Seat

The story of a possessed person in a book who will not hesitate to kill for her goal. Brinny, who lived in obedience to her stepfather, thought it had nothing to do with her. Until she got involved in an incident with the sickly second prince, Lucian. At a moment of crisis, Brinny used her supernatural powers to save Lucian, and was relieved that everything was resolved when she saw that the possessed person, 'Aridna', was captured. However, for some reason, her beloved sister and the first prince died in a carriage accident, and even Lucian, who became her fiancé, began to act strangely. Wasn't the possessed person executed in the past? Is 'Aridna' still alive? Brinny decides to reclaim the empty space left by the disappearance of her precious people. She will fight the possessed person. (Source: KakaoPage, translated)