Joyce has recently married Philip Hammond and moved into his family's mansion on the Caribbean island of St Michael. After a walk on the beach, Joyce is visited by a demon calling himself a French man named Jean Pierre. He appears to Joyce as a “zombie” and slowly begins to demand things from her. After Joyce suffers a nervous breakdown, she is hospitalized and treated for anxiety. The Hammond family feels that Joyce isn't really interacting with a demon, instead she's suffering from fatigue and her new surroundings. When the demon asks Joyce to poison her father-in-law, the book begins to delve into a criminal conspiracy involving a bank and illegal gambling.
Macabre Manor is merely an average Gothic novel with the traditional genre tropes – inheritance, wealthy family, supernatural sprinkles, and a vulnerable female embarking on a dark mental journey. According to Goodreads, Elizabeth Kary used the pseudonym Elizabeth Grayson to author a number of historical romance novels. However, based on my research, I can't verify if this author is the same one that wrote two other Gothic novels in the 1970s for Manor Books – By Demon's Possessed (1973) and Token of Evil (1974). Based on the quality of Macabre Manor, I'm in no hurry to find out.