I put the darkness/the unknown because that scares the absolute **** out of me. I've always been afraid of the dark and I'm 20 years old and still afraid of it (although less so). Something else that ranks equal to the darkness would be losing someone when I had the chance to save them. An example would be if someone I loved (girlfriend, wife, mother, brother, friend, etc.) got into a car crash and I didn't help them when I had the chance, that would torment me until my dying day.
It would really freak me out if my best friend (a zombie clown doll who's a real character when he's in a good mood, but, frankly, takes some gettin' used to) were to be possessed by the ghost of a werewolf while we were visiting the old reptile house in an abandoned zoo on a cold, misty moonless night when the ghost is in his human form: a bloodthirsty chainsaw-wielding serial killer. Yep, that'd do it.