A lot of great elements, and the writing is professional, and the characters are interesting, but the way the story unfolds is bland and boring, devoid of mystery or suspense.
The biggest problem is that the scary event happened far in the past, and while it affected the characters in profound ways, it doesn't seem relevant to much about their lives now. If they needed to revisit the problem to solve some current horror, it could be good, or even if it just started before the event and focused on the buildup and immediate after effects, it might have been an interesting read. As it is, it is very difficult, and if you check on Goodreads, you'll see many people wondering if they should even bother finishing the book. I'd say if you read 1/4th of the book and it hasn't piqued your interest greatly, no need to finish. It doesn't really go anywhere or get any more scary or interesting.