Sudoku.
The original challenge.

Place digits 1–9 once in every row, column and 3 × 3 box. Fixed clues cannot be changed. Use Notes to pencil possibilities; conflicting notes are rejected only when you try them, never hinted by the number selector.

Hard, Extreme and Professional are internal bands selected from a checked puzzle bank using solver-search effort and unresolved cells after simple deductions. They are not an external certification or guarantee of a specific human solving technique.