looking for a horror teen "possessed" movie i watched in the 80's.?

[Rick]

me and my cousin cannot figure this one out. we watched a horror flick back in the 80's on tv. the story goes, a young lady goes to a slumber or teen party at a big house. could be a mansion. she pauses before going in and feels something wrong before entering. she goes in and everything is normal at the party. one by one each teen becomes possessed killing everybody. i remember a part in the movie when someone kills the possessed person and they become possessed. at the end the person left standing is possessed. please help me stump this 20 year question because we talk about this every time we wanna watch a scary movie ;/