SudokuBoku

Evil sudoku

The hardest puzzles that still surrender to technique. Nothing here needs a guess — but you will need patterns most solvers never learn.

Easy Medium Hard Expert Evil Extreme

00:00 Mistakes 0
Puzzle —
Rules
  • Fill every row, every column and every 3×3 box with the digits 1 to 9.
  • A digit can appear once in each of those — never twice.
  • There is exactly one solution, and logic alone will reach it.
  • Rows and columns each take the digits 1 to 9, once apiece.
  • The boxes are irregular shapes, drawn with the heavy outlines.
  • However a region bends, no digit repeats inside it.
  • The colour patches are cages; the small number is what a cage's digits add up to.
  • No digit repeats within a cage — nor in any row, column or 3×3 box.
  • There are no given digits: the sums alone pin down the whole grid.
  • Everything from classic applies: rows, columns and 3×3 boxes take 1 to 9.
  • The two tinted diagonals must each hold 1 to 9 exactly once as well.
  • All the classic constraints hold — rows, columns and 3×3 boxes take 1 to 9.
  • Each of the four shaded windows must also contain 1 to 9 exactly once.
  • The windows sit a step in from the corners, overlapping four boxes apiece.
  • Rows, columns and 3×3 boxes work exactly as they always have.
  • On top of that, two squares a chess knight's move apart may not hold the same digit.
  • Select a square and its knight partners are ringed with a dashed outline.
  • Every row, column and 3×3 box holds 1 to 9, exactly as in classic.
  • Along each grey path, digits strictly climb from the round bulb to the far end.
  • They don't have to be consecutive — each square just beats the one before it.
  • Rows, columns and 3×3 boxes take 1 to 9, exactly as in classic.
  • The digits along each arrow add up to the digit in the circle it comes from.
  • Digits may repeat along an arrow — as long as no row, column or box objects.
  • Fill every row, column and 3×3 box with the digits 1 to 9, as usual.
  • Between squares in the same box you'll find a sign; its point aims at the smaller digit.
  • Every comparison inside every box is shown — a hundred and eight of them.
  • The higher levels hand you fewer digits, not fewer signs. Expert gives you four.
  • Fill every row, column and 3×3 box with the digits 1 to 9, as usual.
  • A white dot between two squares means their digits differ by one.
  • A black dot means one digit is exactly double the other. 1 and 2 are both, and always show black.
  • A boundary with no dot is a clue too — those two digits are neither one apart nor double and half. Every boundary is marked or deliberately bare.
  • Expert hands you no digits at all. The dots and the gaps are the whole puzzle.
  • Fill every row, column and 3×3 box with the digits 1 to 9, as usual.
  • A square with a ring behind it holds an odd digit — 1, 3, 5, 7 or 9.
  • A square with a frame behind it holds an even one — 2, 4, 6 or 8.
  • Most squares are marked with neither, and those are genuinely unrestricted.
  • Every square switches to the fully marked board — one level, gentler than it looks, and two puzzles sharing a grid rather than one.
  • Every row, column and box holds five odd digits and four even ones — so once five squares of a house are known odd, the rest of it is even, marked or not.
  • Solve the 9×9 sudoku as usual — every row, column and box takes 1 to 9.
  • Ten ships are hidden on the same squares: one of four, two of three, three of two and four single squares.
  • Ships are straight, and no two of them touch — not even at a corner.
  • The numbers beside and below say how many ship squares each row and column holds.
  • A ship listed with digits sits on exactly those digits, in order — so the grid is your map.
  • Five 9×9 grids interlock: four corners and a centre, each corner sharing one 3×3 box with it.
  • Every grid obeys ordinary sudoku on its own — rows, columns and boxes each take 1 to 9 once.
  • A tinted box belongs to two grids at once, and only together do the five have one solution.
  • Fill every row, every column and every box with the digits 1 to 6.
  • The boxes are three squares wide and two tall.
  • Nothing repeats in any row, column or box.
  • Put 1, 2, 3 and 4 into each row, each column and each 2×2 box.
  • No digit may appear twice in any of them.
  • Every square can be worked out — no guessing needed.
Keyboard shortcuts
1–9
Place a digit, or toggle a pencil mark
Arrows
Move around the grid
N
Notes mode on or off
Backspace
Clear the square
Ctrl + Z
Undo · Ctrl + Y redo

"Evil" means something specific here

On most sites, evil is simply the label above expert — a mood rather than a measurement. We use it for a precise thing: a puzzle that cannot be finished with any of the patterns on the expert page, but can be finished with an almost locked set or a swordfish. Every puzzle served here has been individually checked against that definition.

That check mattered more than we expected. The raw collection these came from turned out to be badly mixed: a quarter of it genuinely met the bar, while a good deal was easier than our expert puzzles and a large slice was harder than the label suggested. Everything that didn't belong has been moved or dropped, which is why this bank is smaller than it started — and why the word means something.

You never need to guess on this page — if you're stuck, a findable pattern exists, however long the finding takes.

Swordfish

If you know the X-wing, you already understand this — it's the same idea one size up.

An X-wing is a digit confined to the same two columns across two rows. A swordfish is a digit confined to the same three columns across three rows. The rows don't each need all three columns; two of the three is fine, as long as between them the three rows only ever use those three columns.

Three rows will consume the digit three times, and they can only do it inside those three columns. So the columns are fully accounted for, and the digit can be struck from them everywhere else. It works with rows and columns swapped, and the awkward part is purely visual: nine potential squares, rarely more than six actually present, spread across the grid.

Almost locked sets

This is the technique that does most of the work at this level, and it's less known than it deserves to be.

A locked set is n squares in a unit holding exactly n candidates between them — three squares holding only 2, 5 and 8, for instance. They're sealed: those three digits go in those three squares.

An almost locked set is one candidate short of being sealed: n squares holding n+1 candidates. It isn't locked yet, but it is one elimination away. Remove any single one of its candidates and the rest snap into place.

Now take two such sets that share two digits, X and Z, where every X in the first set sees every X in the second. Only one of the sets can supply the X. The other is therefore starved of an option, locks, and is forced to supply the Z. You don't know which — but either way, a Z lands inside one of the two sets. Any square that can see all the Z candidates in both sets therefore cannot be a Z itself.

Finding one without losing an hour

Start with bivalue squares — a square with exactly two candidates is the smallest possible almost locked set, and pairs of them are where most usable examples come from. From there, look at units with three or four empty squares between them holding one more digit than they have squares.

Then check for a shared digit whose positions in the two sets all see each other. That mutual-exclusion link is the rare part, and it's what makes the whole thing work.

How long these take

Considerably longer than expert, and the time is spent searching rather than deducing. An evil grid will typically open normally, stall completely, and then need a deliberate, systematic hunt. Expect stretches where you make no progress at all.

That's the intended experience, and it's worth saying that "no progress" here is not the same as "no path" — the path is guaranteed to exist.

If you want to meet either of these on demand rather than waiting for one to turn up, the technique trainer will find you a swordfish position, or an almost locked set, with everything easier already played out.

If it's still too gentle

Extreme is a different proposition entirely: puzzles where every technique on this site, including everything above, runs out. Those need chains, or a decision to test something and see what breaks.