Suspense is key. If the opening makes the reader wonder what will happen next, it's engaging. For example, 'The door creaked open slowly, and a cold wind blew in, but no one could be seen outside.' This makes the reader want to know who or what is there.
One way is to start with a sense of mystery. For example, describe an unusual sound in the dead of night like 'A strange scratching echoed through the empty hallway, as if something unseen was clawing at the walls.' Another way could be to introduce a spooky setting immediately, such as 'The old, decrepit mansion loomed at the end of the overgrown path, its broken windows like soulless eyes.'
A good horror story opening usually has a creepy setting or atmosphere to draw you in right away. Maybe it's an abandoned house on a stormy night or a dark forest with strange noises.
The opening line 'The man who bought my soul already owned my body.' is quite a great horror opening. It throws you right into a disturbing situation. It makes you question how someone could have their body and soul in such a compromised state. It's a line that grabs your attention and drags you into a world of horror and the unknown.
You could introduce a mysterious character or an unexplained event. Make the reader curious and on edge from the get-go. Also, use vivid and spooky descriptions to set the mood.
The opening paragraph sets the scene, say it's about a dark forest where strange noises are heard. I'd continue the story like this. I entered the forest, my flashlight barely cutting through the thick darkness. Every step I took seemed to echo, as if something was following me. Then, I saw a pair of glowing eyes in the distance. I froze, not knowing whether to run or stay still. The eyes started to move closer, and I could hear heavy breathing. I finally mustered the courage to run. Branches whipped at my face as I sprinted through the forest. I tripped over a root and fell. As I looked up, a large, shadowy figure loomed over me. It reached out with long, claw - like hands.