Horror from 80/90's: gate of hell is opened by the military and demons come out and destroy the world

[Dana]

I don't exactly know when this book was written, I have read it when I was around 15 I guess? I am 35 now, so the book is written at least 15 years ago.

It's about a boy that finds a mysterious fence. Behind this fence he sees a dog like creature. One day he finds out what is happening behind the fence: it's a military base that is trying to open a gate to hell, to use the demons as some kind of war tool. But ofcourse this is not happening and everything goes wrong.

I remember one scene where the boy is on a hill, there is mist all around him. it's the only place that isn't already taken over by the demons. In my mind this was the end of the book, but because it is so long ago that I have read it, I am not sure.

I know this plot sounds just like the mist but it wasn't Stephen King. ;) I hope someone knows which book it was, because I was way to young when I read it and I am really curious if it was really as disturbing as I thought at that time.

[Dana]

I'm not sure, because it was from the library, so all the books are hardcover there. The cover was with trees on it, but I'm not 100% sure of that.