An ordinary sudoku, plus one rule: the two long diagonals, ruled across the grid, also hold 1 to 9 exactly once. Rows, columns and boxes still do too.
Diag is a free daily diagonal sudoku, the variant often called Sudoku X, from Mind Loft. Each day gives you a 9×9 grid with one rule added to the usual three: each of the two long diagonals, corner to corner, also holds 1 through 9 exactly once. Rows, columns, and 3×3 boxes still do too, and those two extra houses do enough of the work that the board can print fewer digits than an ordinary sudoku needs.
There is no arithmetic: the two diagonals are tinted and ruled across the grid, and selecting a square on one lights the whole line as a peer, the same way its row, column and box light up. A diagonal is a house like any other, so it can hand you a hidden single, and the centre square, which sits on both, sees more of the board than any other square. Every board has a single solution you can always reach by logic, with no guessing, and on every board the diagonals are load-bearing rather than decorative: read as an ordinary sudoku, the same clues would fit more than one grid. Wrong entries are never flagged, so spotting your own slips is part of the puzzle, and a clean solve scores a perfect 10 with the daily leaderboard decided on the clock.
A new board drops every day at midnight Eastern; weekdays print fewer digits as the week goes on, from 26 on Monday down to 16 on Saturday, and the Sunday Edition prints fourteen. No app, no signup, play free in your browser. Want the rest of the family? Try Suds, the classic 9×9, Knight, the anti-knight sudoku, Cages, the killer sudoku, and Polka, the kropki with no numbers at all.