Evil Sudoku
The extreme tier — puzzles that provably require X-Wings, XY-Wings, and forcing chains. Not fewer clues. Harder logic.
What earns the grade
Most "evil" or "extreme" sudoku is graded by clue count or solver timing — neither says anything about the logic you'll face. Stillgrid grades by required technique: our solver works every puzzle like a human, simplest move first, and a puzzle only reaches the top tier when it cannot be finished without chain-based techniques. We call that tier Nightmare — same beast other sites call evil, extreme, or expert — and every puzzle served at it has been verified to deserve it.
Prefer a different intensity? Every puzzle on Stillgrid is graded on the same honest ladder, from Easy up.
A certified evil puzzle
Proof over promises. Our grader solved this exact puzzle in 75 steps, escalating only when every simpler technique ran dry — and it had to reach for X-Wings, XY-Wings, and forcing chains before the grid gave in. The full path:
| 1 | 6 | |||||||
| 8 | 3 | 2 | ||||||
| 7 | 6 | |||||||
| 9 | 6 | 8 | 2 | |||||
| 2 | 1 | 3 | ||||||
| 4 | 9 | |||||||
| 5 | 4 | 6 | ||||||
| 7 | 4 | 9 | ||||||
| 3 | 8 |
24 givens · graded Nightmare · 75 solver steps
- Naked single× 50
- Hidden single× 7
- Naked pair× 2
- Hidden pair× 1
- Pointing pair× 6
- X-Wing× 2
- XY-Wing× 2
- Forcing chain× 5
Have a puzzle you suspect is evil? Grade it yourself — same ladder, same honesty.
How to solve one
- Exhaust the quiet moves. Fill every naked and hidden single first. Evil puzzles hand you a few before the wall — take them, they anchor everything after.
- Mine the pencil marks. Note every candidate, then squeeze: naked and hidden pairs, pointing pairs. Each elimination is small; together they expose the pattern the puzzle hinges on.
- Hunt the patterns. When simple logic stalls, the puzzle is demanding a pattern: X-Wing rectangles, Swordfish across three lines, or an XY-Wing pivot. Scan candidate pairs row by row.
- Follow the chains. The hardest grids need chains — follow a candidate's implications from cell to cell until an assumption contradicts itself. That contradiction is the solve.
The full ladder, with examples, lives on the learn pages.
Do you have to guess?
No — and that is what separates an honestly-built evil puzzle from a cheap one. A genuine evil puzzle always has a path made of pure logic; you just have to climb higher up the ladder to find it. When singles and pairs run dry, you don't start guessing — you look for a pattern: an X-Wing, an XY-Wing, and at the hardest step a forcing chain.
A forcing chain is the move that feels like guessing but isn't. You take a cell with two candidates, follow what each choice forces, and keep only the eliminations both paths agree on — a conclusion that holds whichever candidate is true. It's proof by cases, not trial and error. So if you've ever wondered how to solve an evil sudoku without guessing, that's the answer: forcing chains, walked step by step. Every puzzle we grade Nightmare has been checked to yield to exactly this kind of logic — grade one yourself to see the path.
Common questions
What is evil sudoku?
Evil sudoku is the hardest tier of sudoku — puzzles that cannot be finished with basic techniques alone. A genuine evil puzzle forces advanced patterns like X-Wing, Swordfish, XY-Wing, or forcing chains. The honest test is which techniques a puzzle requires, not how few clues it has: removing clues makes a grid emptier, not necessarily harder.
Is evil sudoku the same as extreme or expert sudoku?
The names blur into each other — sites variously call their hardest tier evil, extreme, expert, or fiendish, and there is no standard. On Stillgrid the top grade is called Nightmare, and it is earned, not asserted: a puzzle only grades Nightmare when our solver provably needs chain-based techniques to finish it.
Are Stillgrid's evil sudoku puzzles free?
Yes. Evil (Nightmare) puzzles are free to play, with no signup needed to start. Each one is generated fresh and verified by the grader to require the techniques its grade claims.
How do you solve an evil sudoku without guessing?
You never have to guess. When the simple techniques stall, an evil sudoku still yields to logic — you escalate to patterns like the X-Wing and XY-Wing, and to forcing chains. A forcing chain tries both candidates of a two-option cell and keeps only the eliminations both branches agree on, so the result is proven either way. Every Nightmare-tier puzzle on Stillgrid is verified to be solvable this way, with no guessing.
Is evil the hardest level of sudoku?
On Stillgrid, evil maps to Nightmare — the top of an honest, technique-based ladder, reached only when a puzzle provably needs chain-based logic. There is one rung beyond it: a grid that defeats every known technique grades as stuck rather than Nightmare. The famous 'world's hardest sudoku' puzzles land there, and we'd rather say so than overstate a grade.