Expert Sudoku Puzzles (Nightmare Level)

Updated 23 August 2026

Nightmare is Suuudokuuu’s expert tier — what most other apps would call very hard or expert. A Nightmare board carries just 25 clues out of 81 cells, and it is the tier where bounded patterns stop being enough: every board here has been checked against a solver that already knows every fish and every wing up to the W-Wing, and is only labelled Nightmare because that solver stalled. Finishing one needs chain reasoning — X-Chain, XY-Chain, simple coloring and AIC, the alternating inference chain that generalises the other three.

What makes a puzzle Nightmare

A chain follows an alternating sequence of strong and weak links between candidates, starting from one end and reasoning outward until the two ends prove an elimination no matter which of them turns out to be true. X-Chain does this with a single digit; XY-Chain walks a path of bivalue cells instead, changing digit at every step. Simple coloring assigns two alternating colours to a network of strong links on one digit and clears any candidate that can see both colours. AIC folds all of them into one technique, mixing digits and link types along a single continuous chain. The difference from a fish or a wing is not subtlety but shape: a wing is a fixed arrangement of three or four cells you learn to recognise, while a chain has no fixed size and has to be built move by move. Nightmare is capped at AIC — anything that still resists is beyond what the generated tiers will hand you.

How hard is it, honestly

Our sample of 40 Nightmare boards measures SE (Sudoku Explainer) 4.5 to 7.3, with the AIC as the most common hardest step. That upper end sits in the same territory as our 17-clue Hell tier, which is the clearest evidence on the site that clue count and difficulty are separate facts: Nightmare hands you eight more givens and asks for the same reasoning. The measured tables live in our sudoku difficulty rating guide. Qualitatively, this is where difficulty stops being about speed and becomes about holding an entire line of implication in your head without losing track of it.

Where to go next

Cleared a few Nightmare boards? Hell Sudoku is the last tier, built from a bundled 17-clue corpus. Step back to Hard Sudoku if fish and wings still feel unreliable. Prefer paper? Download the printable Nightmare sudoku booklet. Browse the technique index, the how to play guide, every tier on the Sudoku difficulty levels hub, or head home.