Responses
Nice theory.. and it seems like she is able to target the next body she wants to take over or may be it was a bluff for the Duke.. it could also be that the Royal family kept books of blackmagic and witch craft despite it being banned by the temple. Since the princess was bedridden and her illness had no cure, all she could do was read various books either to appease her boredom or to find a obscure cure that most people can't. However the TL, doesn't match.. she was already dead in the 1st TL when the FL was being killed right?
I have a feeling that the body of the princess is a vessel to possibly a previous lover or even wife of the man in the locket. Because they said the princess was acting a lot different after getting sick, she could’ve died and possibly a became a vessel. Another thing I feel like supports it, is that she knows about dark magic that was lost over a century ago.