Easter Monster Sudoku — Play It Online, With the Full Solution

Updated 15 October 2026

Easter Monster is a 21-clue sudoku from the solving community’s lists of the hardest known puzzles, famous less for a number than for a result: Bob Hanson and Dave Marans published a complete logical solution for it in 2008. Play the grid below, print it, or compare their human solution with the engine’s walkthrough further down.

Clues

21

Published SE rating

none we can attribute

Suuudokuuu rater

8.5+, past every implemented technique

Play Easter Monster in the appPrint Easter Monster — PDF with the solution

Where Easter Monster comes from

Its givens form an almost symmetrical pattern, and that symmetry is what made a human-sized proof possible at all. SudokuOne calls it perhaps the most difficult sudoku ever solved without a computer solver. It is not part of the Suuudokuuu Infinity corpus.

Sources: Hanson and Marans, “Easter Monster” and SudokuOne, “A General Sudoku Logic”. The other record grids are on the hardest sudoku puzzles page.

The Easter Monster solution, step by step

Suuudokuuu’s source-available engine solves Easter Monster in 116 steps, reaching for the cheapest technique that still makes progress, exactly as it does when it rates a puzzle. 19 of those steps are forcing chains: the engine assumes a candidate, follows its consequences until something breaks, and removes the assumption. At 2 points even that runs dry — no technique the engine implements moves the grid — and the walkthrough says so, fills the cell with the fewest candidates from the verified solution, and carries on with logic from there. Every other step is a named deduction you can check by hand.

Along the way the walkthrough uses Full House, Naked Single, Hidden Single, Pointing Pair, Box Line Reduction, Naked Pair, Naked Triple, Hidden Pair, X-Wing, Finned X-Wing, Sashimi X-Wing, XY-Chain, Simple Coloring, AIC and Nishio Forcing Chain.

The engine’s route is not Hanson and Marans’ route. Theirs is one long, hand-built argument that exploits the symmetry; the engine has no notion of symmetry and grinds forward one cheapest step at a time. Replay any of its steps in the step-by-step solver to see it on a live board.