Pretty spooky story so far.
As a JLPT N4~N3, Wanikani Level 18 Japanese learner, I read the raw alongside the translation and only had to heavily rely on the translation for a couple rough patches.
Overall, I found most the vocab wasn't too bad, but the dialect/slang/casual speech patterns were hard for me. That probably just means that I needed it though!
Overall, not terribly difficult for a mature story, definitely be ready for gore though.
I don't really feel like I understood the point of the story but it was good study material. At Wanikani level 20, you'll see a lot of kanji that you just learned like 夢 and 悪夢. There will still be a lot of lookups, but hopefully you'll pick up on some things pretty fast; a lot of them are pretty useful like 先輩 and 久しぶり. It's only 44 pages so you could probably do it in a session or two at this level, pretty bite sized.