The Dark (1980)
The Dark by James Herbert

Night falls.

It came like a malignant shadow with seductive promises of power.

And somewhere in the night... a small girl smiled as her mother burned... asylum inmates slaughtered their attendants... in slimy tunnels once-human creatures gathered.

Madness raged as the lights began to fade and humanity was attacked by an ancient, unstoppable evil.

Quotes...

James Herbert "When I was writing the Dark, about halfway through I thought, 'Wait a minute, I've been here before - this is The Fog, a re-run!' And what I realised was there was so much left over from The Fog, so much to explore in that total madness, that I hadn't finished the book. Plus I was making The Dark a 'supernatural' book, adding a lot more to it. So I was actually fulfilling the promise of The Fog."

- James Herbert, from "Haunted by Rats" by Dave Hughes, Skeleton Crew II, 1988