Bloody Summer (Verano Sangriento) 35 years old

The story of a group of teenagers heading to a big ol' house that is owned by an old relative. They want to spend a nice Summer weekend there swimming in the pool, playing pranks on each other, and contact the supernatural via a Ouija board. It's all fun & games till people start getting killed. Could it be someone in the house committing these horrible murders? Or is it something supernatural doing it??