When designing your horror adventure, you should spend a bit of time selecting your enemies and locations to ensure that it's all thematically appropriate for a true dash of Hi-Octane Nightmare Fuel. After all, your set dressing is vitally important for creating the right vibe.
So where do you get your inspiration for what might work?
Well, often times your adventure begins with an idea that you can work from. Perhaps you want to throw your characters against vampires. Or perhaps you have a world that reacts to the victim's mind. Or you just know you want to run an adventure in the woods. Maybe you have no clue at all as to what you want to throw at people. Let's look at each of these possible options.
The Critter Idea. Vampires. Brainstorm what you think makes vampires scary and figure out ways to emphasise that. If your list reads mental violations, corruption, rape analogies, and physical invulnerability you'll want to run them differently than if your list includes things like undeath, unstoppable hunger, physical invulnerability, and super speed that keeps you form running away. Read up any canonical representations of vampires in your system (i.e. check the Bestiary in Pathfinder or the various bloodlines in Vampire: the Requiem) to see what fits best for your tale. Think about how you could tinker with the rules to make a variant monster or bloodline to make it fit your image better. Perhaps brainstorm a list of possible locations that would best showcase your monster to its full advantage - either by contrasting it against a place completely unsuited to it, combining it with weather or other environmental hazards / obstacles, or by emphasizing it against a place that really fits it.
The Concept. You don't know the monster or the environment but you know the cause. Perhaps it's a curse on the land. Or a nightmare world that plays on your fears. Or an infectious disease that transforms the people and the place. Any which what way, your best bet is to look at the characters and see how you can reflect them into the concept. This is fairly obvious with a psychoactive world like Silent Hill that plays off your character's hates, fears, and sins (list them) but it also works for infections that warp a place or curses. It'll be more personal to them if it affects a place they know. If you're tossing up between a few different places / monsters, list out their pros and cons. A hospital will provide a different experience than a prison which would be different to a lighthouse, in terms of hazards and equipment.
The Place. You know where you want to place your adventure, whether a working prison or an abandoned temple to Saranrae, but you don't know what the players will be facing there. Figure out why you picked that place. Is it because its secluded or urban? Is it because of certain hazards you have in mind? Is it because of what it represents to the players or their characters? Figure out all the reasons for that location, then list them out and brainstorm ways to really put it all together. If you want a prison because it's a place of corruption (or the temple because it's not supposed to be) than figure out ways to emphasise that. Perhaps the prisoners are becoming twisted by their violence into something inhuman. Or perhaps it's more about claustrophobia and living in a cage, in which case, perhaps have something prey on a captive audience.
In my case, I had a creature (a Malefactor Earthbound), some pre-established enemies, and I just needed a place. I'd already established that she was somewhat out of the way, in Nairne, and in an earlier campaign set in the same place I'd established a cave network established by previous foes that earlier characters had cleared out. This gave me a location with history. It also happened to be near Nairne AND the players had recently been told to avoid it. It played to the Malefactor's strengths, being underground, and prevented the characters from being able to escape easily. I then figured out how the Earthbound might adapt the location and what sort of hazards might naturally remain there.
So yes, hopefully all that advice will help you when it comes to figuring out your
Oh, also, you can find a list of the other articles in this series here.