The Shuttered Room (1967)

"There are some doors that should never be opened..."

In a small island off the American coast, the Whateleys live in an old mill where a mysterious bloody being creates an atmosphere of horror. After her parents get killed by lightning, young Susannah is sent to New York by her aunt Agatha, who wants her to avoid the family curse. Years later Susannah, now married, persuades her husband to spend a holiday in the abandoned mill. Once on the island, Susannah and Mike soon find themselves exposed to the hostility of a gang of thugs led by Ethan, Susannah's brutal cousin.