Nightmare Castle/Amanti D'oltretomba (1965)

In the Nineteenth Century, In Italy, the wealthy Muriel Arrowsmith is married to Dr. Stephen Arrowsmith in a loveless marriage. They live in an ancient castle and Stephen spends most of his time researching in his laboratory in the basement. When Stephen tells that he would travel during the night, Muriel meets her lover David in the greenhouse. However, the servant Solange warns Stephen that returns unexpectedly and sees them both making love. He locks them in chains in the dungeon and tortures them to death, keeping their hearts in a tank and cremating their bodies. Stephen expects to inherit Muriel's fortune, but she had changed her will in favor of her half-sister, Jenny that is interned in a mental institution under the treatment of Dr. Dereck Joyce. However, Stephen plots a scheme to marry Jenny, drive her crazy and keep Muriel's fortune for him, with the support of his lover Solange.

IMDb: https://www.imdb.com/title/tt0060097