About Evil Sudoku
Evil Sudoku is our sparsest level: every puzzle starts with only 23 or 24 clues — fewer than any other level on the site, and close to the known minimum of 17 for a valid Sudoku.
With so little information on the board, early progress is slow and the middle game can be a long hunt for one elimination. Every evil board still has exactly one solution, reachable without guessing.
Every evil puzzle is checked by our step-by-step solver before you see it: it must need techniques up to beyond (chains) — and at least Swordfish & XYZ-Wing. So the label always matches the challenge. Stuck? The hint button names the technique and walks you through it.
Best for
Expert solvers looking for the sparsest, most demanding grids.
Techniques you will use
- Chains & forcing logic
- XYZ-Wing
- Swordfish
- Unique rectangles
Surviving an evil Sudoku
- Start by placing everything singles can give you, then fill in a complete candidate grid. Do not skip cells.
- Work one digit at a time looking for fish patterns (X-Wing, Swordfish) — sparse boards hide them in plain sight.
- When patterns run dry, try short chains: follow 'if this cell is X, then…' a few steps and look for a contradiction.
- Notes never count as mistakes, but entries do. Only commit a digit when you are certain.
Want the full method? Read our Sudoku tips and strategy guide and the step-by-step X-Wing technique explainer.
Frequently asked questions
How many clues does an evil Sudoku have?
Every evil puzzle on vSudoku starts with 23 or 24 given numbers. We generate until a board lands inside that range, so the label always matches what you get.
What is the fewest clues a Sudoku can have?
Seventeen. Mathematicians proved in 2012 that no valid Sudoku with a unique solution can have 16 or fewer clues.
Is evil Sudoku solvable without guessing?
Yes. Every board has exactly one solution and can be solved with logic, though some positions call for advanced techniques such as chains or wings.
Is evil the hardest level?
Yes — it is the sparsest level on vSudoku and the best test of advanced technique.