Dementia 13 (1966)

John Haloran has a fatal heart attack, but his wife Louise won't get any of the inheritance when Lady Haloran dies if John is dead. Louise forges a letter from John to convince the rest of his family he's been called to New York on important business, and goes to his Irish ancestral home, Castle Haloran, to meet the family and look for a way to ensure a cut of the loot. Seven years earlier John's sister Kathleen was drowned in the pond, and the Halorans enact a morbid ritual in remembrance. Secrets shroud the sister's demise, and soon the family and guests begin experiencing an attrition problem.