Night of the Damned.

Jean Duprey and his wife Danielle are famous journalists with a special interest in unsolved mysteries. They receive a request of help by an old noble friend of Jean, Guillaume de Saint Lambert, who is about to die by an unknown illness. He lives, obviously, in ancient castle with his wife and the servants. Soon Duprey discover that his friend's wife is a witch (originally burnt in the 18th century) and that Guillaume is under her wicked influence. She practices the art of black magic and by making die slowly all members of the Saint Lambert family she gains eternal beauty. When Duprey breaks the marble casket containing the ashes of the witch she shows her real aspect before dying.