Really Really Hard Sudoku

If the hardest puzzle in your daily paper is a warm-up, this is for you. Every puzzle has exactly one solution and is solvable by logic — no guessing — and is graded by the hardest technique it actually needs. Seven tiers run from Easy to Nightmare in both Classic and Killer Sudoku. HINTS THAT TEACH • Hints get more specific one step at a time: the technique, then the digit, then where to look • Lessons for every technique, with a worked example • "Explain step by step": watch a chain or forcing argument unfold link by link, on an example or on your own board • Mistake hints narrow down to the box, then the cell PLAY YOUR WAY • Normal and candidate entry, unlimited undo and redo • Auto Pencil marks that remember the ones you cross out • Killer cage helper and remaining sums (optional) • Check and reveal when you need them — tracked in your stats, never forced on you STATS • Best and average times, streaks, clean solves, and the techniques you've beaten • Share a solve when you finish a Nightmare No accounts, no ads, no tracking. Works offline. Plays on iPhone and iPad, with full hardware keyboard support.