All Sudoku Variants
9×9 · Misc

Fog of War Sudoku

Also known as Fog Sudoku

The grid starts hidden under fog that lifts cell by cell as you place correct digits, revealing the puzzle as you go.

5 3 9 7 1

An example Fog of War puzzle: only a few cells are lit; the fog lifts around each correct digit you place.

Fog of War Sudoku hides the grid under a layer of "fog". Only a few starting cells are visible; the rest are dark. As you place correct digits, the fog lifts around them, revealing more of the puzzle — so the grid uncovers itself as you solve. Underneath the fog it is an ordinary Sudoku with a single logical solution.

The rule: correct digits clear the fog

Every time you place a digit that is right, the fog clears from that cell and the cells around it, exposing whatever they hold. A wrong guess reveals nothing (and, of course, isn't part of the solution). Below, a lit cell holds a 5 while the fog still covers its neighbours.

5

A single lit cell (5) surrounded by fog (dark cells).

A worked example, step by step

Start with almost everything fogged and one visible clue.

5

Step 0. One clue (5) is lit; everything else is under fog.

Step 1 — place a correct digit. Using the visible clues and the ordinary rules, you work out that the cell to the right must be a 2 and place it. Because it is correct, the fog clears around it.

5 2

Step 1. The correct 2 clears the fog around it, revealing new cells (light).

Step 2 — use what's revealed. The newly-lit cells may contain fresh clues, which in turn let you place more digits and clear yet more fog. The solve becomes a chain reaction: each correct placement opens up the next part of the puzzle, and the whole grid gradually comes into view.

How to play it well

The golden rule is never guess. A wrong digit doesn't clear anything and just clutters the board; worse, on some versions it can be hard to tell a guess from a deduction later. Solve exactly as you would an ordinary Sudoku — from certainties only — and treat the fog as a reward that confirms you are on the right track. Because the puzzle underneath is a normal unique Sudoku, pure logic always suffices.

Solving strategy

  • Only place certainties. Fog rewards correct digits; guesses do nothing useful.
  • Follow the clearings. Each revealed cell may carry a new clue — read it before moving on.
  • Work outward from lit areas. The solve spreads from where the fog has already lifted.
  • Treat it as ordinary Sudoku. The hidden grid has one logical solution; standard techniques apply.

Common mistakes

  • Guessing to reveal cells. A wrong digit clears no fog — deduce, don't gamble.
  • Assuming fog means "no clue". A fogged cell simply hasn't been revealed yet; the puzzle underneath is complete.
  • Forgetting the grid is normal. Under the fog it is an ordinary Sudoku, solvable by logic alone.
  • Ignoring the usual rules. Rows, columns and boxes all still apply as you uncover the grid.

Can I play it here?

Not yet — Fog of War Sudoku isn't one of the puzzle types SudokuStreak generates for now. But you can learn its rules above, and there's plenty here to sink your teeth into meanwhile: