Evil Sudoku Online
Free 9×9 puzzles at the hardest tier we generate — the toughest techniques, pure logic.
Evil puzzles are chain territory — no guessing required. Read the Forcing Chains walkthrough on the wiki and learn to follow a candidate to its logical conclusion.
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All-TimeWhat is evil sudoku?
Evil sudoku is the hardest difficulty we generate on Sudoku247Online. It uses the standard 9×9 rules — fill every row, column, and 3×3 box with the digits 1 to 9 — and every puzzle has exactly one solution you can reach by pure logic. What makes a puzzle evil is not how few numbers it starts with; it's the depth of technique the solution demands. Each grid is built so that you can't make progress with simple scanning alone — you have to spot advanced patterns to break it open. Every puzzle is freshly generated, so you'll never see the same board twice, and finishing one earns the leaderboard XP bonus.
Evil vs Expert: how much harder?
Evil is a clear step up from Expert. On Expert, careful single-cell logic and a few intermediate moves usually carry you through. Evil routinely forces longer chains of inference and the harder patterns below before the grid gives way. The time jump is real: a confident solver who clears Expert in 8 to 10 minutes will often spend 30 minutes or more on a single Evil puzzle. That's the point — these are for solvers who want a genuine cognitive workout, not a quick win. If Evil is wearing you out, step down to Expert Sudoku and build up from there.
Techniques you'll need
Expect to lean on advanced patterns the easier tiers rarely require: the X-Wing, swordfish, XY-Wing, naked and hidden triples, and patient candidate chains. Most Evil grids fall only when you stack three or four of these together. Guess-and-check won't help you — every Evil puzzle is uniquely solvable by logic, but the deductive path is buried deep, so the work is finding it, not gambling. A few practical tips: use Notes liberally to track candidates, hit Show Candidates whenever you stall, and don't be shy about tiering down to Master if a grid stops being fun.
Evil sudoku FAQ
What is evil sudoku?
Evil sudoku is the hardest difficulty tier we offer. It follows the standard 9×9 rules, and every puzzle has a single solution reachable by logic. What sets it apart from easier tiers is the advanced techniques you need to make progress — not the number of starting clues.
Is evil sudoku harder than expert?
Yes. Expert puzzles can often be cleared with single-cell logic and a few intermediate moves, while evil puzzles demand a deeper toolkit of chains and fish patterns. The same solver who finishes Expert in 8 to 10 minutes will routinely spend 30 minutes or more on an Evil grid.
How many clues does an evil sudoku have?
Clue count does not decide difficulty. A 17-clue puzzle can be easy and a 30-clue puzzle can be brutal — what matters is the hardest technique the clue placement forces you to use. (17 is the proven minimum number of clues for any uniquely solvable 9×9 grid, shown by McGuire, Tugemann, and Civario, but that's a property of all sudoku, not a defining feature of evil.)
Can evil sudoku be solved by logic without guessing?
Yes. Every evil puzzle we generate has a unique solution you can reach by pure deduction — you never need to guess. The catch is that the chains of reasoning are long, so the challenge is finding the next logical step, not picking a number at random.
What techniques do I need for evil sudoku?
Plan to use advanced patterns such as the X-Wing, swordfish, XY-Wing, naked and hidden triples, and forcing chains. Most evil grids require several of these stacked together. Our strategy guides walk through each one if you want to practise before diving in.
How long does an evil sudoku take to solve?
Far longer than expert. Many solvers take 30 minutes or more on a single evil puzzle, and that's completely expected. Finishing one under that time puts you among the strongest solvers anywhere online.
Source: Mathematics of Sudoku (Wikipedia) — the 17-clue minimum proof.