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Nightmare Fuel: Abarat
From the first book, we have...
The John brothers. One man... and six tiny heads growing out of the horns on his head. The book is illustrated, so even if you don't want to picture it, you see it. Hello, Body Horror!
On that note, Shape — The Dragon. A one-legged man who walks just fine on the stump of his missing foot... and has swords embedded in the flesh of his back, which he can pull out and put back in as well. Yeesh!
The Stitchlings, zombie-like minions filled with mud—and occasionally random internal organs—stitched together from random bits of old rotting flesh, by an insane old woman who never sleeps, and who once sewed her grandson's lips together for daring to say the word "love."
The brothers on the Time Out of Time, who have no faces — just random facial features on crablike legs, that crawl around their otherwise blank faces.
And of course, there's the Big Bad himself, whose face is surrounded by a vat of liquid in which electrical, pulsing nightmares swim. They come from tubes embedded in his skull.
Then you get to the second book, and you add to this...
Leeman Vol, one of the Big Bad's henchmen that lost his nose to a spider and wears a crude leather nose in place, and is able to speak to insects because of his three pointy-teethed mouths. Not helping is that Vol has an insect-infected head. He has so many lice and ants and things living on him that when Shape hits him, some bugs actually fall off.
The Sacbrood, a voracious, highly adaptable and eternally reproducing kind of insect that inhabits the pyramids of Xuxux. The picture may not be very definite, but then again, depending on the strenght your imagination, the scene where they kill Mendelson Shape may be scarier.
Carrion's nightmares are pictured even more terrifying, including a scene right in the first chapter when they suck the fear out of a rather innocent man.
Then there's the picture of Christopher right before he dies. A freakin' skull, screaming out of a dark abyss!