The Redeemer: Son of Satan!

A group of people are trapped within their old high school during their 10th high school reunion with a psychotic, masked preacher who kills them off for their sinful lives they have made for themselves. The film follows six individuals - John, Cindy, Terry, Jane, Roger, and Kirsten - as they try to find a way out of the building after being invited to a reunion at their alma mater, only to be stalked and killed one by one by the mysterious killer known as The Redeemer. The film's release in 1978 marked it as a historically significant slasher film, predating other iconic horror movies such as Halloween.