Diabolical Sudoku

What makes a Sudoku Diabolical here

Every puzzle on Sudoku Challenge is solved by our grader step by step, the way a careful person would: at each step it uses the simplest technique that works. A puzzle's level is the hardest group of techniques it cannot be finished without.

So a Diabolical puzzle is one that every Extreme pattern together cannot finish. A few, under 2% of them, go beyond all 28 techniques our solver knows; they still have exactly one solution. Its score sits between 8,000 and 9,999 points, shown before you start.

Our Diabolical puzzles have 21 to 32 clues, 25 on average, about as many as our Hard ones. What counts is the techniques.

The techniques you need

First the ones from the easier levels that Diabolical puzzles use most: Naked Single, Hidden Single, Locked Candidates, Naked Pair, XY-Wing and W-Wing. Then the Diabolical techniques, with how often our grader used each one across our 10,000+ Diabolical puzzles:

Counted October 2026 over every puzzle on the site. More in Sudoku by the Numbers.

How to solve a Diabolical Sudoku

1. Clear everything the easier levels can, down to the X-Wings and XY-Wings, and keep your candidates exact: one missing candidate breaks every chain.
2. Look for cells with two candidates that link up, each sharing a digit and a row, column or box with the next. When the chain’s two ends share a digit, any cell that sees both ends cannot hold it: an XY-Chain.
3. Look again at the fish with one extra cell, the fin: a Finned X-Wing still eliminates from the cells that see both the X-Wing’s lines and the fin.
4. Look for small groups of cells holding one more candidate than cells (almost locked sets): two of them linked by a digit make an ALS-XZ.
5. Stuck for long? Next Step in the game shows the next move and why.

Diabolical puzzles across the site

Questions about Diabolical Sudoku