Confident solvers who think they’ve got Sudoku down cold will have to think again, because these are absolutely the most fiendishly difficult, devilishly baffling Sudokus ever devised…and the biggest, too.  Created by hand in Japan—from Nikoli, the company responsible for the puzzles’ popularity in that country—the collection includes megasudokus on a 25 x 25 grid, designed to give even the sharpest minds a workout. True to its title, it’s the ultimate challenge, and one that millions of Sudoku-obsessed fans will find irresistible. And for total newcomers brave enough to high-dive in at the deep end, there’s a quick tutorial to get them started.