Whether you call them hard, difficult, fiendish, devilish, migraine or black belt, these are puzzles for people who need more of a challenge.
Times Killer Sudoku 1-9
Nine volumes of previously unpublished puzzles for hard-core Su Doku addicts. The puzzles use the same 9x9 grid but there is a double challenge - it is also necessary to ensure that the outlined cubes add up to the same number.
Black Belt Sudoku By Michael Rios. Taking a lead from karate--another Japanese art--each title is graded by color: White Belt for easy, Green Belt for medium level, Brown Belt for hard, and here is Black Belt for the super-tough solvers. Contains 300 puzzles.