Killer Sudoku is a fun and challenging variant of Sudoku. If you love Sudoku, why not give Killer Sudoku a try?
As with Sudoku, the objective of Killer Sudoku is to fill the grid with the numbers 1 to 9 such that each row, column and nonet (3x3 group of cells) contains each number only once. In addition, a Killer Sudoku grid is divided into cages, shown with dashed lines. The top-left corner of each cage shows the sum of the numbers within it. The same number cannot appear more than once in a cage.
There are ten puzzles included for free, and thousands available for download. The puzzles are graded into difficulty levels, all excellent quality and with a unique solution.
The app and puzzles are from the maker of https://www.dailykillersudoku.com.