What makes Evil Sudoku so hard?
It comes down to one number: 18. That's how many digits an Evil puzzle starts with, against 36 for Easy — exactly half. Every empty cell you don't fill in leaves more room for the others to stay uncertain, so the whole grid resists you for longer.
With so few givens, the obvious moves dry up almost immediately. On an Easy grid you can place number after number just by scanning. On Evil, you'll often stare at a full board of pencil marks with nothing forced anywhere — and that's normal. The puzzle is still perfectly logical; the logic is just buried deeper.
One thing never changes, though: there is always exactly one solution, and you never have to guess. However thin the start, an Evil puzzle can be solved by pure deduction. If you're guessing, there's a technique you haven't spotted yet.
Techniques you'll actually need
The beginner tools — naked singles and hidden singles — still matter; they're how you turn each breakthrough into a run of placements. But on their own they'll stall fast. Evil is where the advanced patterns earn their keep:
- X-Wing Sudoku Technique Explained — One digit confined to the same two columns across two rows (or vice versa) — eliminate it from the rest of those lines.
- Forcing Chains — Follow both values of a bi-value cell; if both branches force the same digit somewhere, that conclusion is certain.
Don't worry if these take practice to spot — they're supposed to. The reward is that moment when an X-Wing you almost missed suddenly clears three cells at once and the grid starts moving again. If you want to build up to it, the Sudoku School walks through every technique with worked examples.
Common mistakes on Evil grids
- Guessing when you're stuck. It feels faster, but one wrong guess can send you ten moves down a dead end. There's always a logical next step — look for it.
- Skipping pencil marks. On Easy you can hold candidates in your head. On Evil you can't — mark every candidate, because the advanced patterns only become visible once they're all on the board.
- Chasing one number. When a region won't budge, switch digits or switch units. A hidden single for a 7 is often sitting right next to the 3 you've been fighting.