Night of the Demons (2009)

Angela throws a Halloween party in the Broussard Mansion, which has been uninhabited for fifty years. Legend has it that many years ago six people disappeared during a Halloween party, but that doesn't concern the partygoers who pay $10 a head to get in.

After the cops arrive to break up the party, Angela, drug dealer Colin, and a small group of friends find themselves stuck at the mansion because the outside gates are locked.

They head back into the mansion and while exploring the basement discover a secret room in the basement which has skeletons lying on the floor. When Angela attempts to steal a gold tooth, the skeleton bites her. Bad news because the bite makes Angela into a demon, even though she usually appears normal.

The demons need to possess seven bodies before daylight and coincidentally, there are seven people left in the building. From this point forward, the movie consists of the demons trying to kill everyone. You'll spend the remainder of the movie wondering why they didn't just climb over the fence once the killings began.