It doesn't get any harder? Well, actually it does! In volumes 27 and 32, Stefan Heine, team captain of the two-time Sudoku world champion Germany, believed he had developed the most difficult killer Sudokus possible. In an international comparison, this was indeed true: Heine's puzzles are by far the hardest in the world. However, this book raises the bar even higher. Killer Sudoku ultrahardcore - puzzles for which even the author takes forever to solve each of these 500 masterpieces!.