Killer Sudoku combines standard 9x9 Sudoku rules with dashed cages. Every row, column, and 3x3 box still needs the digits 1 to 9, but each cage also has a target sum. Digits cannot repeat inside a cage, so a tiny arithmetic clue can control a whole corner of the board.
The variant became popular because it turns the quiet logic of Sudoku into a puzzle about combinations. A 3 in two cells can only be 1 and 2. A 17 in two cells can only be 8 and 9. Those little facts start the solve before any normal Sudoku clue appears.
Start with cages that have very few possible combinations, especially tiny totals, large totals, and single-cell cages. Then layer those cage limits onto normal row, column, and box restrictions.
Good Killer Sudoku solving is not mental arithmetic alone. The strongest moves usually happen where a cage crosses a box edge or squeezes a row into only one possible combination.
Classic Sudoku often begins with visible givens. Killer can begin almost empty, which makes the first breakthrough feel earned. Once the cage structure starts talking, the grid can unlock in a chain of clean deductions.