- Diabolical puzzles require fish patterns (X-Wing, Swordfish) or multi-cell chains
- Nightmare / Extreme puzzles often need ALS chains, forcing chains, or trial-and-inference
- Every advanced technique is still pure logic — never guessing
- The fastest path: eliminate everything simpler first, then apply advanced techniques to the remaining candidates
- Keeping clean, complete pencil marks is mandatory at this level
What Diabolical Actually Means
A puzzle rated "Diabolical" (or similar names like "Very Hard" or "Extreme") has been verified to require at least one technique beyond Naked/Hidden Pairs and Locked Candidates. The most common requirements are fish patterns and XY-Wings. Before applying any advanced technique, always exhaust all simpler ones — a stall that looks advanced is often just a missed pair.
Fish Patterns: X-Wing and Swordfish
Fish patterns work by finding a digit confined to a fixed set of rows within a fixed set of columns (or vice versa).
| Pattern | Rows Used | Columns Used | Effect |
|---|---|---|---|
| X-Wing | 2 | 2 | Eliminates digit from all other cells in those 2 columns (or rows) |
| Swordfish | 3 | 3 | Eliminates digit from all other cells in those 3 columns (or rows) |
| Jellyfish | 4 | 4 | Eliminates digit from all other cells in those 4 columns (or rows) |
Chains and Alternating Inference Chains (AIC)
A chain is a sequence of cells linked by strong and weak relationships. An Alternating Inference Chain (AIC) alternates between strong links (a digit must be in one of two cells) and weak links (a digit can be in one or both of two cells). When a chain forms a loop or connects two cells that see each other, you can make eliminations based on the chain's endpoints.
The Forcing Approach
For true Nightmare/Extreme puzzles, some solvers use trial-and-inference (sometimes called "forcing chains"): assume a digit goes in a specific cell and follow the logical consequences. If that assumption quickly leads to a contradiction, the digit is eliminated. This is still logic — not guessing — but it is computationally intensive and requires very clean candidate tracking.
Practical Advice
Work through the full techniques library systematically before attempting Diabolical puzzles. The progression: Easy techniques → Medium → Pairs → Locked Candidates → Fish → Chains. Skipping steps makes advanced puzzles feel impossible when they are actually just one missed pattern away from progress. The Expert and Daily Challenge pages let you practice at a controlled pace.