The 1974 Bob Clark slasher starring Olivia Hussey, Keir Dullea, Margot Kidder.
So a sorority (of course) is having their Christmas party when the psycho murderer who killed his family in (you guessed it - the same house before it was a sorority house) comes home for christmas, havign escaped from the mental hospital.
Many credit this as the original teen slasher pic whch established "the rules" which are fully explained in Scream.
