Creatures: The Big Night (Creatures #2)

In a post-apocalyptic New York City where the toxic fog of the first Big Night turned most of the population into drooling zombies and drove pockets of survivors underground to hunt for food and live in hiding, a group of bickering teenagers and a raving old man set out to rescue a little boy from the clutches of the mysterious creatures that hold him and try to prevent the second Big Night from descending on the beleaguered city and destroying what's left of the sane human population.