Evil sudoku – today's 4 hardest puzzles

Permission to spend an hour on one puzzle

Evil sudoku is not built for speed. It is built for the pleasure of spending thirty minutes — sometimes well over an hour — drawing a single thread of logic through the grid. Up to expert level the question is which technique to use; at this level the question is which techniques, in what order, combined how. Confirmed cells barely appear in the opening, and you spend long stretches staring at a board that is almost entirely candidates. And yet there is always an exit that pure logic can reach. Whether you can keep believing in that guarantee is the whole art of playing at this level. A dead end is not a failure; it is the step before meeting a piece of logic you haven't met yet. Once you can see it that way, you already live here.

What "AI Escargot" taught the world

Sudoku has its legendary grids. The best known is AI Escargot, published in 2006 by the Finnish mathematician Arto Inkala, with just 23 given digits and a reputation for requiring several advanced techniques chained in a particular order. The interesting part is that even this puzzle is understood to be solvable by a chain of inferences rather than brute force. Difficulty, in other words, lives less in the scarcity of clues than in the depth of the reasoning and the complexity of the combination. Our evil selection follows the same philosophy: never merely spiteful, always rewarding to anyone willing to think deeply.

From single techniques to chains

The typical evil grid unfolds like this. Locked candidates and the pair family trim the notes, and still nothing is confirmed. You go looking for an X-Wing or a Swordfish, find one, and a handful of candidates disappear. Only then does the shape of a Y-Wing or an XY-chain become visible. Each technique is groundwork for the next; every elimination is a plant for a later payoff. Which is exactly why accurate candidate notes are the foundation of everything. If any individual pattern still feels shaky, review the shapes in the technique guide and put in time at expert level until single techniques land reliably.

Before you start: environment and record-keeping

It is worth setting yourself up properly. Being interrupted mid-inference — a question, a notification — collapses the stack of assumptions you've built in one second. Find a quiet block of time, and on a tired day, skip it without guilt; knowing when not to play is part of playing well. The other habit worth building is a note of where you got stuck: which position, which technique you were hunting, and why you couldn't find it. Reviewing that after you finally solve the grid exposes your search habits with startling clarity: "I see column-wise locks but I keep missing row-wise ones." An evil puzzle carries so much information that one grid can teach more than ten easy ones. Chew a single puzzle to the bone rather than racing through many; the strongest solvers count insights, not completions.

What to do when it really won't fall

Hours with no progress is unremarkable here. Before you retire hurt, try this analysis. Suspect not a missed technique but an error in your notes. A single stale or missing candidate makes an existing pattern invisible. Recount every unit from 1 to 9 and audit the notes. If that changes nothing, enter your current board into the sudoku solver. It verifies that your work so far is consistent, and simply knowing "everything up to here was right" is a solid platform for another attempt. Rather than revealing the whole answer, take a hint for one cell and then work backwards to understand why that cell was forced — used this way the solver is a teacher, not a defeat.

A new challenge every day

Four of the daily 20 are at evil level. If today's beat you, four more arrive tomorrow, and the ones you left behind stay in the archive for a rematch once you're stronger. There is no luck in this game: every win and every loss is entirely on the board in front of you. As opponents go, few are so honest. Good luck.