Final Girl: A Knock at the Door

Nothing like a reunion at the quiet and secluded Wingard Cottage. Everyone is having a great time. The booze is flowing and the party has begun. Then comes a knock at the door. A quick glance around the room tells you everyone who's supposed to be here already is... You approach the door and slowly open it, peeking around the edge of the door. No one is there. But someone was... the question is, who?