I've already made a topic describing this, a few months ago. HOW do you make a creature fearsome. Say, like Nazgûls. Nazgûls, in the books, were shown as fearsome from the reactions of characters. How would you write that? How do you make something scary?
''Nazgûls, in the books, were shown as fearsome from the reactions of characters.'' here's your answer.
I think you answered your own question, at least in part. Character reactions to something can have a big impact on whether its scary or not. Another thing to remember, you're "Showing" people what's happening. If you show it to them in a way that plays of basic human fears, like fear of the dark, pain, the unknown, and the unknowable, it'll be alot more scary than just describing the creature. You kind of have to understand what fear is, and what inspires it.
Well, you don't need a quote. What you need is to picture your own reaction to something as frightening as what you're trying to create. So when you're faced with your worst nightmare, what do you do? Do you cry? Scream? Wet your pants? Faint? All of the above, in one order or another? It's all about portraying FEAR.
Along that worst nightmare part, most people don't stop to think about what that really is. There was something when you were young that scared the living bejesus out of you. It was a dream that still haunts you today, and I remember mine, even though it was almost 35 years ago. i remember the inability to act, to run, to get away, even though the creature was getting closer. The worst part was that I knew the terrain and how to escape, but it was still gaining on me. No way to escape, no way to kill the monster, no way to find protection, and the emotions with all of that are in those dreams. If you can remember what those nightmares were about, and convey, in a conversation to someone how you felt, then you can do the same for one of your creations. Dark or evil, unstoppable, or that way by conventional means. The Aliens from the old movie franchise had a lot of those characteristics. Hard to kill, killed the host while being born, reproduced quickly, and their death might also kill you. Can't reason with them, appeal to their compassionate side, or their human hearts. they didn't have one. I hope this helps.
You can use an element based on fear. For example, when I was a little boy, my fear is the darkness. I can not see what is in the dark, and that made me very nervous. So how about a monster that can remain hidden in the darkness where you can't see it coming for you. And if the monster is unexplainable, that can also trigger fear. Most people are afriad of what they can't understand. "WHAT THE $%&@ IS THAT?"
The trouble is, tapping into what you fear isn't very helpful. All you could manage is to frighten yourself, and after several proofreading/editing passes, you won't even get a shiver from it. Each reader will have different fears, and different specific triggers for those fears. The more specific you get in your writing to try to fire off those triggers, the more of your readership you will miss entirely. So what you want to do is steer the reader to find his or her own triggers. You send them in a general direction, and nudge them to line up their own triggers.