I'm trying to remember the title of a book I read back in the late 1970's (I think, maybe the early 80's). The central character was a young-ish woman who had received an invitation to spend time at the mansion of someone she didn't (or barely) knew. She and her husband went and met a handful of other people who were also visiting. My recollection is that people died (or were killed) one by one. At the end, the host told the woman that she was the chosen one to take over his supernatural powers. Included in this would be a very long life. She would choose "apprentices" (my word, I don't remember what the book said) from whom she would choose her successor when the time came, just as she had just gone through. She then chose her husband as her first apprentice.
Within a year or two of reading the book, a movie was made that was fairly true to the book.