What Is Evil Sudoku?
Search for evil sudoku and you land on a word that sounds like a puzzle type - something with extra rules, like Killer or X-Sudoku. It isn't. Evil is a difficulty label, not a variant. The grid is an ordinary 9×9 Sudoku with ordinary rules; what makes it evil is how little of it you are given to start with.
Where the name comes from
There is no standards body for Sudoku difficulty. Every site invents its own ladder and its own names for the rungs, which is why the top of the range is called something different almost everywhere you look: Evil on one site, Expert, Extreme, Master or Diabolical on others.
"Evil" is the name used by Web Sudoku, one of the longest-running Sudoku sites on the web, which describes itself as offering "four levels from Easy to Evil" - Evil being the hardest of the four. The label stuck, and plenty of solvers now use "evil" as shorthand for the hardest setting a site offers, whatever that site happens to call it.
So when someone searches for web sudoku evil, they are usually not looking for a particular website - they are looking for a puzzle at that level of difficulty. The name travelled further than the site did.
What "Evil" means here
Because the label is not standardised, the only honest way to describe a level is by the number it starts you with. On SudokuStreak, Evil means 18 given digits out of the 81 cells:
We deliberately don't quote anyone else's numbers here, because we haven't verified them and a difficulty label on its own tells you nothing. If you want to compare two sites honestly, compare their clue counts - not their adjectives.
Eighteen is one above the floor
Eighteen is not an arbitrary choice. A proper Sudoku must have exactly one solution, and it turns out you cannot go below 17 clues and keep that property: in 2012 a team at University College Dublin proved by exhaustive computer search that no valid 16-clue Sudoku exists. Seventeen is the true floor, and thousands of 17-clue puzzles are known.
An Evil grid at 18 therefore sits one single clue above the mathematical minimum. That is why the openings dry up so fast: there is barely any redundancy in the grid to give you a second way in. The full story is in How Many Clues Does a Sudoku Need?
Does "evil" mean guesswork?
No - and this is the part that trips people up. A harder puzzle is not a less logical one. Every Evil puzzle here has exactly one solution and that solution is reachable by pure deduction. If you find yourself guessing, there is a pattern on the board you have not spotted yet.
What actually changes as you climb the ladder is which patterns you need. On Easy, scanning for singles carries you the whole way. On Evil, the simple moves run out within a handful of placements and you need the techniques that work on candidates rather than on digits:
- Snyder notation - a disciplined way to pencil in candidates without drowning the grid in numbers
- X-Wing and Swordfish - eliminations that come from the shape of a candidate across rows and columns
- XY-Wing - three cells that between them force a digit out
- Forcing chains - following a candidate's consequences until a contradiction settles it
The Sudoku School works through all of them, easiest first.
Evil, Extreme, Diabolical - are they the same?
Roughly, but not exactly, and you cannot rely on the name. Two sites both offering "Extreme" may hand you grids that differ by four or five clues, and a well-constructed 22-clue puzzle can be nastier than a plain 18-clue one - clue count sets the floor of the difficulty, while the arrangement of those clues decides the rest.
The practical advice: ignore the adjective, look for the clue count, and if a site doesn't publish one, judge it by whether the puzzles ever force you to guess. A puzzle that does is not hard - it is broken.
Where to start
If Evil is a step too far today, that is the normal experience rather than a failure. Expert at 22 givens asks for the same family of techniques with a little more room to breathe, and it is the honest place to build them up before dropping to 18.
Where to practise this
This technique starts paying off at Evil, which is where puzzles first stop yielding to anything simpler. Reading about a pattern and spotting it on a live grid are different skills - the second one only comes from playing.