Anyone Know this Horror movies name ?

Hello, I watched a Horror movie in the late 1970s and
have wanted to know its name ever since. The plot is
3 friends, one of whom is a Heires, go to her mansion
here we meet a creepy Maid and I believe a Butler.
One night the Maid comes to the Girls/Heires room
and takes her to presumably the dungeon where she
inherits the family curse or is cursed. In the end, I
believe the male friend kills the creature thus killing
his friend the Heires. I remember feeling sorry for her
as she was tricked or duped into becoming the creature
or beast. PLEASE help, Thank you !