What makes an effective horror opening sequence?

An opening to a horror sequel is very important as it sets the scene and it should be clear to the viewer that the film they are watching is a horror. The opening should have iconography used because if it’s a slasher film then it should show what the antagonist is slashing with. The opening should have engaging characters so it attracts the viewer. The opening should include either a monster fodder who is going to get killed off early or even the last-girl. The opening should start off with non-diegetic eerie music to build tension.