"Slashers are a particular subgenre of horror films that are not always original. The formula can be very simple; somebody in mask comes and kills everybody one by one. However, recently, as obvious from "Happy Death Day" films, it's always possible to put an original spin into it. "Texas Chain Saw Massacre", "Black Christmas" and particularly, Carpenter's "Halloween" created a certain slasher formula that many other films followed.
They don't always have great acting or clever plot but the shocking deaths are used effectively to give us the viewers a tension and we might get interesting characters (and some dumb ones as well) through the storyline, up to the final which is usually our hero facing with the villain. If we're lucky, we might get a good twist too. Villains are often mysterious and scary."