Evil Sudoku Puzzles

Evil Sudoku is the final frontier. Sometimes called Master Sudoku, these puzzles have as few as 17–22 starting clues and require techniques that go far beyond what any other difficulty level demands. Where expert puzzles ask you to spot visual patterns like X-Wings, evil puzzles require you to reason through multi-step logical chains — tracing “if-then” consequences across the grid before arriving at a single elimination.

If you’ve mastered expert Sudoku and solved every X-Wing and Swordfish you could find, evil is the ultimate test of your deductive reasoning.

How Evil Differs from Expert

The jump from expert to evil represents the single most significant shift in cognitive approach across all Sudoku difficulty levels.

Expert Uses Pattern Recognition; Evil Uses Chain Reasoning

Expert Sudoku introduces grid-spanning techniques like X-Wing, Swordfish, and XY-Wing. These techniques have recognizable visual shapes — a rectangle, a Y-shape, a staircase pattern. Once you know what to look for, you can scan the grid and spot them relatively quickly.

Evil Sudoku’s defining techniques — chains and coloring — don’t have a fixed visual shape. Instead, they require you to follow a logical thread: “If cell A is 7, then cell B can’t be 7, which means cell B must be 3, which forces cell C to be 9…” and so on, sometimes across six or more cells, until you reach either a placement or a contradiction.

This is fundamentally different from the pattern-matching used at expert level. It’s more like solving a logic proof than finding a picture in a grid.

The Numbers Tell the Story

Coming from Hard: A Word of Caution

If you skipped expert and came straight from hard, evil puzzles will contain techniques you’ve never encountered. We strongly recommend mastering X-Wing, Swordfish, and XY-Wing at the expert level first. Evil puzzles use all of these techniques in addition to chains and coloring — they don’t replace them.

Techniques That Define Evil Sudoku

Simple Coloring

Coloring is a technique for analyzing a single candidate number across the grid. When a candidate appears in exactly two cells in a unit (a “conjugate pair”), those two cells have an either-or relationship — one must be true and the other false. By assigning two colors and tracing these pairs across the grid, you can find contradictions or confirmations that eliminate candidates.

How it works: Pick a candidate and color one cell blue, the other green. Follow conjugate pairs through connected units, alternating colors. If two same-colored cells end up in the same unit, that color is false — eliminate the candidate from all cells of that color.

Alternating Inference Chains (AICs)

AICs generalize coloring beyond a single candidate. They trace logical implications through cells connected by strong links (exactly two positions for a candidate in a unit) and weak links (candidate appears in a cell). By alternating strong and weak links, you can build a chain that proves a specific candidate must or cannot be placed.

AICs are the single most powerful solving tool in evil Sudoku. A skilled solver who understands chains can crack almost any evil puzzle.

Forcing Chains

When a cell has two candidates, you can trace what happens if each candidate is true. If both possibilities lead to the same conclusion (for example, both eliminate candidate 5 from cell D4), that conclusion must be true regardless. This “try both and see what’s forced” approach is logically valid — it’s not guessing because you’re proving the outcome is inevitable.

When to Use What

A practical hierarchy for evil puzzles:

1. Exhaust all lower techniques first. Singles, pairs, triples, pointing pairs, X-Wings, Swordfish, XY-Wings, XYZ-Wings. These still handle 60–80% of an evil puzzle.
2. Try simple coloring. Fast to apply and often sufficient for the first breakthrough.
3. Look for short AICs. Chains of 4–6 links are common and manageable.
4. Use forcing chains as a last resort. These are time-consuming but always produce results when shorter techniques fail.

A Solving Workflow for Evil Puzzles

Evil puzzles demand discipline. Here’s a structured approach:

1. Build a complete, accurate candidate grid. This is non-negotiable. Every technique at this level depends on perfect candidates. Take extra time here — it saves far more time later.

2. Apply techniques in difficulty order. Work through naked singles, hidden singles, naked pairs, hidden pairs, pointing pairs, naked triples, hidden triples, X-Wing, Swordfish, XY-Wing, and XYZ-Wing before attempting chains. Always use the simplest technique available.

3. When you hit a wall, try coloring. Pick the candidate number that appears most frequently in conjugate pairs. Color one cell and follow the chain. Look for contradictions (two same-color cells in one unit) or eliminations (a cell that sees both colors).

4. If coloring fails, build AICs. Start from bivalue cells (cells with exactly two candidates) and trace implications. Focus on short chains first — most evil puzzles don’t require chains longer than 8 links.

5. Track your logic. When using chains, write down (or mentally track) each step. One logical error in a chain can corrupt the entire puzzle. If something feels wrong, verify the chain from the beginning.

6. Re-scan after every breakthrough. Chain techniques often produce cascade effects — a single elimination from a chain can unlock a dozen placements through simpler techniques.

The Psychology of Evil Sudoku

Evil puzzles test more than technique — they test temperament:

• Patience over speed. Evil puzzles can take an hour or more. Rushing leads to candidate errors that compound into dead ends.
• Comfort with uncertainty. You’ll often stare at a grid for several minutes before seeing a path forward. This is normal. The breakthrough usually comes from a fresh perspective, not from trying harder.
• Systematic persistence. When coloring doesn’t work, try a different candidate. When one AIC leads nowhere, start from a different cell. Evil puzzles always have a logical path — you just need to find it.
• No guessing. The golden rule applies more than ever. Every evil puzzle on Sudokupulse can be solved through pure logic. Guessing introduces uncertainty and makes it impossible to backtrack reliably.

Frequently Asked Questions

What’s the minimum number of clues in an evil puzzle? The theoretical minimum for a unique-solution Sudoku is 17 clues. Our evil puzzles sometimes approach this limit, which is part of what makes them so challenging.

How is evil different from expert? Expert requires grid-spanning pattern techniques like X-Wing and Swordfish. Evil requires chain-based reasoning — tracing logical implications across multiple cells to make deductions that no pattern technique can find.

Can every evil puzzle be solved without trial and error? Yes. Every puzzle on Sudokupulse has a unique solution reachable through logic alone. Chain techniques may feel like guessing because you’re exploring hypothetical scenarios, but they’re actually rigorous logical proofs.

I can solve expert puzzles but evil seems impossible. What should I learn first? Start with simple coloring on a single candidate number. It’s the most accessible chain technique and often sufficient for your first evil solves. Our advanced strategies article covers this in depth.

How long do evil puzzles take? Experienced solvers typically spend 45–90 minutes. Some particularly difficult evil puzzles can take over two hours. This is normal — evil Sudoku is meant to be a marathon, not a sprint.

Explore our full technique library to review every strategy from naked singles to XYZ-Wings. If you need a breather, try our daily puzzle or warm up with hard Sudoku or expert Sudoku.