The MC of the book has DID (dissociative identity disorder) & 1 of the “headmates” has taken a position as caretaker at a castle isolated on an island, another headmate, after a long period of dormancy has to deal with this 

Given the mental health condition it’s tough to find a reliable narrator (multiple POVs)&Much of the mystery plays out inside the MC’s mind (but there’s plenty of weird stuff at the castle). 

I have never read anything like it.