# Killer Sudoku X Rules
Killer Sudoku X is a twist on the Killer Sudoku theme, borrowing the concept of the 'X' constraint that Sudoku X added to Sudoku.
In addition to the rules of Killer Sudoku, that are explained in detail on our killer sudoku rules page, there is one more rule to consider: the diagonal must also contain each number used in the puzzle once and once only.
Thus there are many things to consider with a killer sudoku X puzzle and the hardest bit is remembering all the constraints of a valid puzzle. It comes with practice but be warned - these puzzles are for the serious puzzle fan!
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