What's a good scary prank?

Okay so I'm throwing a party at my house at night. We are gunna set up a projector in my back yard and watch scary movies. It's gunna be pretty dark. So I really want to scare them. And I literally go all out. So I need a legit prank that will scare the s**t outa them! We juniors in high school so it has to p**s your pants scary