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I think its from the 90's. It starts out with this yuppie and his friend going home from work(I think they're lawyers) and they see a guy slapping a girl on the sidewalk. So they break it up and the guy pulls out a knife and says "you don't know what you're messing with". So one of the yuppies takes the girl home, she becomes his girlfriend, admits that she has powers, says that she used to belong to a witch cult that is trying to track her down, then in the end she turns out to be evil and he is lured into a ritual sacrifice and his own friend kills him.
A scene I remember vividly is the guy and the girl take a look out the window of a house and there is a crowd of faces covering the whole of the window.