The Velvet Vampire (1971)

Lee and Susan, a married couple, accept an invitation from the mysterious Diane LeFanu to stay at her secluded estate in the desert. At first, their hostess seems merely alluring and enigmatic, but unease grows as both husband and wife become the focus of her seductions. Unaware that Diane is a centuries-old vampire, the couple find their visit turning into a tense and dangerous encounter.