Fiendish Sudoku
Fiendish is the fourth Master Sudoku tier on TheSudoku.com. It selects puzzles with Sudoku Explainer ratings from 7.0 up to, but not including, 8.0, giving experienced players a specific challenge above Veteran and below Supreme.
Play Fiendish Sudoku free online
The game above opens this Master tier directly. Play in your browser without a download or registration. Use candidate notes to record possibilities, undo to revisit a move, and hints when you want help studying a position. Choose New Game to start another puzzle from the same tier.
The rules remain the same: place the digits 1 to 9 once in each row, column and 3×3 box. If you need a refresher, read the Sudoku rules. For a gentler practice session, choose Easy Sudoku or Medium Sudoku.
What Fiendish means on this site
Sudoku difficulty names vary between websites and puzzle books. Our Fiendish label refers to the 7.x rating band within Master; it is not a claim that every puzzle called “fiendish” elsewhere has the same difficulty. The numerical range helps you understand which part of our collection you are choosing.
If you are comfortable with Veteran Master Sudoku, this is the next band to explore. Expect to invest attention in candidate relationships and to justify removals carefully. If the logic becomes hard to follow, practising a related technique on a lower rated puzzle can be more productive than pushing through with guesses.
Practise chains without losing the logic
The forcing chains guide is a useful study resource for understanding how candidate assumptions lead to consequences. Keep track of the exact proposition at each step: which candidate is true or false, and which Sudoku constraint forces the next result. A chain is useful only when every link is valid.
Review 3D Medusa for another way to study connected candidates, and compare simpler coloring and loop techniques. These topics can improve your reasoning even when a particular Fiendish puzzle has a different solving path. The rating selects the grid, not a prescribed sequence of moves.
Make progress one verified elimination at a time
Resist the urge to chase several speculative ideas at once. Start with one promising relationship, work through it carefully, and check the conclusion against the current notes. Once a candidate is removed, inspect the affected row, column and box for a simpler follow-up. Difficult puzzles still contain ordinary singles and subsets.
Take a break if you find yourself repeating the same scan without learning anything new. Return to the advanced Sudoku strategies when you need a fresh approach. For a higher rating range, try Supreme Master Sudoku; for a wider choice, visit the Master Sudoku overview or the daily Sudoku challenge.
Explore the five Master Sudoku levels
Choose a rating band for your next game. Each level opens its own playable Sudoku page.
The bands use Sudoku Explainer ratings. They describe difficulty, not a time limit or a required list of techniques.
