The Book of Lost Things/YMMV

Complete Monster - The Crooked Man. Quite apart from holding a massive gallery of past victims and special tortures, his method of Staying Alive is utterly repulsive: he talks a child in our world into sacrificing someone they hate to him- usually a younger brother or sister; he gets feed upon their heart and their life for as long as they would have lived, while the corrupted child is crowned king or queen... of a world built out of his or her own nightmares. For good measure, the child's tormented by the reality of what they've done, and will eventually discover that all his or her attempts to atone for this crime by ruling justly will come to nothing, because they have no power of their own. Eventually the king or queen will grow old, the victim's life force will begin to fade, and the Crooked Man goes looking for another child that's prepared to have their unwanted siblings horribly murdered... and he's been doing this for centuries.
- Another rotten character would be the huntress: bored with her usual game and finding humans too fragile to keep up the entertainment, she's taken up grafting the heads of human beings onto animal bodies- through a technique she learned from three surgeons she happened to abduct and torture. Because adults don't adjust well to the shock, the recipients of this treatment are children, who generally spend the last tortured minutes of their lives running through a dark forest in an unfamiliar body before being shot dead, partially eaten, and their preserved remains hung from a wall. Holy shit.
High Octane Nightmare Fuel - Much of the book is filled with this.
What Do You Mean It's Not for Kids? - It's a story about a twelve year old boy who visits a fairy tale land... and is filled to the brim with High Octane Nightmare Fuel.