What is your general grammar level? Is it on par with WK, or are you more advanced when it comes to grammar?
For horror, I don’t think I know of any beginner resources, sadly. Once you’re at an intermediate level (say, N3 or the like) you could dive into normal books for natives; Japan features an enormous amount of genre book prizes and so of course there is also a Horror Book Award where you can start diving into the genre if you have no idea where to find interesting material otherwise.
Other than that, I can only suggest building your reading ability with less interesting material, e.g. check out the Absolute Beginner Book Club or the Beginner Book Club; you can also read previous picks and use the discussion threads to clarify questions you might have. Other people go by NHK Easy News or Satori Reader or Graded Readers, the former being geared towards Japanese children while the latter two are made with learners in mind.
No matter what route you take, enjoy your reading journey!
I agree with this, but will also add that I recall Satori Reader (a graded reader app) having some “creepy” stories that were alright and at least one longish true crime account of a serial killer. I haven’t used the app in a long time so it’s possible they’ve added more since. Because it’s a graded reader with read alongs, tappable grammar explanations, word definitions and etc it might get you over the hump into native material where you’ll have a surplus of choice if anything.
Only horror I really read was Tsunekawa Kotaro’s two books of short stories, 秋の牢獄 and 夜市, but I really enjoyed them. I would say they are more like scary stories than horror depending on your definition, but they are published under Kadokawa Bunko Horror, at least. Though they are definitely intermediate level books, leaning a bit on the simpler side.