Fog of War Sudoku turns the puzzle you know into an expedition. The 9x9 board starts hidden under a layer of fog — only a small seed area is visible. Every correct digit you place clears the fog around it, revealing new givens, new empty cells, and, hidden somewhere in the mist, 2–4 surprise constraints: killer cages, kropki dots, thermometers, or arrows. The standard row, column, and box rules apply from the first move to the last, but you discover the rest of the puzzle as you solve it.
What is Fog of War Sudoku?
Fog of War Sudoku is a classic 9x9 Sudoku wrapped in a discovery mechanic borrowed from strategy games. Underneath the fog sits a complete, unique-solution puzzle — every row, column, and 3x3 box must contain the digits 1 through 9 exactly once. What makes the variant special is what you can't see: most of the grid, most of the givens, and a handful of extra constraints are covered at the start. The fog is purely visual — it never changes the underlying solution — but it changes how you experience the solve, turning each correct placement into a small reveal.
Fog of War Sudoku Rules
- Standard Sudoku rules: Each row, each column, and each 3x3 box must contain the digits 1 through 9 exactly once — including in cells still hidden by fog.
- Start at the seed reveal: The puzzle opens with one clear area — a seed cell's 3x3 box plus its full row and column. Everything else is fogged.
- Correct digits clear fog: Placing a correct digit lifts the fog around that cell. Wrong digits reveal nothing.
- Surprise constraints: Each game hides 2–4 extra constraints — killer cages, kropki dots, thermometers, or arrows — that appear as the fog clears and must be satisfied by your solution.
How the Fog Reveal Works
At the start of every puzzle, one seed cell anchors your entry point: its 3x3 box, its entire row, and its entire column are revealed, giving you a cross-shaped window of visible cells and givens. From there, the reveal is earned one digit at a time. Place a correct digit and the fog around that cell dissolves, exposing neighbouring cells — and any givens or constraint markings sitting in them. Place a wrong digit and the fog stays put.
The Surprise Constraints
Hidden under the fog, every Fog of War puzzle carries a small set of extra rules — 2 to 4 per game — drawn from four familiar variant families. When the fog over one of them clears, its markings appear on the board and its rule is in force. Each behaves exactly as it does in its home variant:
Killer cages
A dotted-line cage with a small target sum. The digits inside must add up to the target and cannot repeat within the cage — the same rule as Killer Sudoku. A freshly revealed cage is often the key that unlocks a fogged region, since sums sharply limit the possible digits.
Kropki dots
Small dots on the border between two cells. A white dot means the neighbouring digits are consecutive; a black dot means one is exactly double the other — see the full Kropki Sudoku guide. Note that in Fog of War, the "no dot means no relationship" deduction only applies once both cells are out of the fog.
Thermometers
A bulb-and-line shape whose digits must strictly increase from the bulb to the tip, exactly as in Thermo Sudoku. Even a partially revealed thermometer is useful: its visible cells already carry position bounds.
Arrows
A circled cell with a path: the digits along the path add up to the digit in the circle, repeats allowed — the rule from Arrow Sudoku. A revealed circle caps every cell on its path, fogged or not.
Solving Strategies
1. Work the frontier
The cleared area's edge — the frontier — is where progress lives. Cells on the frontier belong to rows, columns, and boxes that are partly visible, so they inherit constraints from the digits you've already placed. Solving frontier cells pushes the fog back and feeds you new information, creating a steady solve-reveal-solve rhythm.
2. Never guess into the fog
Every Fog of War puzzle has a unique solution reachable by logic from the visible cells at every moment. Guessing is doubly punished here: a wrong digit clears no fog, and it can quietly poison deductions in an area you can't yet verify. If you stall, re-scan the frontier — there is always a next logical move among the revealed cells.
3. Treat each revealed constraint as fresh evidence
A new cage, dot, thermometer, or arrow emerging from the fog is information you didn't have a move ago. Pause and apply its logic immediately — cage sums, dot relationships, thermometer bounds, and arrow caps often crack cells that plain row-column-box logic couldn't touch.
4. Lean on standard technique
Underneath the fog this is still Sudoku. Naked singles, hidden singles, pointing pairs, and the rest of the toolkit from our solving techniques library work on whatever portion of the board is visible — the fog only limits what you can see, not what the logic can prove.
Mini Fog of War (6x6)
Mini Fog of War brings the same hidden-board mechanic to a compact 6x6 grid using the digits 1 through 6 with 2x3 boxes. The fog reveal works identically, and each game hides 2–3 surprise constraints instead of 2–4. Because the grid is small, the frontier moves fast and the whole board opens up in minutes — and it's free to play, making it the perfect first taste of the fog. Read the full Mini Fog of War guide to get started.
Tips for Beginners
- Mine the seed area first. The revealed box, row, and column intersect — digits placed there constrain each other heavily. Fill everything you can before pushing outward.
- Expect the unexpected rules. Don't commit to a digit near freshly cleared fog without checking whether a new cage, dot, thermometer, or arrow just appeared beside it.
- Partially revealed constraints still count. Half a thermometer or two cells of a cage already narrow candidates — use them before the full shape emerges.
- Slow is fast. One verified digit that clears fog beats three speculative pencil marks in the dark.
- Start on Mini Fog of War. The free 6x6 version teaches the reveal rhythm and the surprise-rule habit on a grid you can finish in minutes.
- Never guess. The fog only lifts for correct digits — guessing gains you nothing and risks everything.
Fog of War Sudoku is the rare variant that changes how solving feels without changing what solving is. The logic is pure Sudoku plus a few familiar constraint types, but the gradual reveal turns every puzzle into a small story: an opening foothold, a growing frontier, a twist when a hidden rule surfaces, and a satisfying final sweep as the last fog clears. Once you've felt a board open up under your own deductions, an ordinary fully-visible grid may seem almost too generous.