Killer sudoku, also called sum sudoku, have no given numbers. Instead, there are sum cages—extra boxes with numbers that are the sums of the numbers inside those boxes.
Like the normal sudoku rows, columns and boxes, no number can be repeated in the sum cages.
Example: The cage sum 4 can only be the sum of 1+3, not 2+2. A cage of three cells with the sum 6 can only be 1+2+3, not 1+1+4.
Killer sudoku is a SudokuMix favorite. There are many different solving techniques to discover, and the puzzles are often very satisfying to solve.