The hybrid child is the product of human-demon parents. It is the key between the human and demon dimensions which are otherwise bound behind locked doors to prevent the human and demon joining, and one of the few ways to open the doors to the endless worlds is the book. And the only hybrid child within a radius of ten thousand worlds capable of reading the book is a legend by now, told like bedtime stories in a world used like Earth's United Nations, dubbed Ghehenna. The only world capable of hiding a hybrid child.