One of the scariest things that a person can encounter is the thought that a serial killer may be after them. There are many plot ideas that you can base around the idea of a serial killer. For the adventure to work properly, the players must be able to figure out who the serial killer is, evade him or discover why he is killing people. You can have the serial killer go after teenagers, women, children or even elderly people. Try to add a twist to the story, such as mental instability or the fact that he kills people he believes to be dangerous. Murder mysteries where the killer ends up disturbed or possessed are also scary.
Another popular horror category is supernatural horror stories. There are many ways to create horror adventure ideas based on supernatural events. Some of the most popular are ghosts coming back from the dead and haunting a place or person, demon possessions, cursed items or people, or other communications with the supernatural world. These ideas are always frightening because no one can prove that these things do not exist. Stick to one main supernatural event so the players don't get confused. If the players are supposed to solve the possession of Emily Smith, don't throw in a cursed object that causes people to die.
Probably the most important idea to remember for a horror adventure is that sights and sounds are scarier than almost anything else. To make an adventure truly scary, there should be enough visual and audible clues to get the hearts of the players racing. For computer games, the sounds are easy enough to add. Scary sights and sounds are harder to use with in-person horror adventures, but they really add to the feeling. You can use ghost howls, clanking chains, children screaming, strongly altered voices and scratching sounds to increase the horror feeling. Make sure to have dark areas with sudden flashes of light to increase the visual creepiness of the area.
