Jacob, Rachel, and Clarisse must overcome obstacles throughout the novel that you and I would never even dream of having to face. Not only must they deal with a world in utter chaos, but they are relentlessly pursued by the very ones that created the madness in the first place.
These enemies appear to most of the people in the book to be human. To Jacob however, they are far from it. With his ability/ handicap, he perceives them only as black vaguely human shaped things. They lack all of the colors of the life-force that we come to know as the ‘aura.' That is not the worst of it.
As the story progresses, we discover that They are not the true enemy, only ITS appendages in our world. They are the hands, feet, eyes and ears of IT, the ultimate enemy that all of our heroes must one day face. An enemy that can and has reduced planets to fist sized rubble.