4
x 2 x 3  5 x x x  x x x x  x x x x
x 6 x x  x 2 x x  x 8 x 3  4 x x x
x x x x  x 4 8 x  x x x x  x x x x
x x x x  x x x x  x 2 x x  x x 6 x

x x 8 x  x x 4 x  x x x x  x x x 1
x x 5 x  3 x x x  9 x x 1  6 x x x
x x x x  x x 6 1  x x x x  x x x x
x x 1 9  8 x x x  x x 4 x  x x x x

x x 7 x  x x x x  x x x x  x x x x
2 x x x  6 x x x  3 x 7 x  x x 5 x
x x x x  4 8 x 5  x x x x  x 6 x x
3 x x 8  1 x x x  6 x 9 x  2 x x x

x x 6 x  x x x 8  x x 1 x  x x 2 x
x x x x  x 1 x x  x x x x  x x x x
x x x x  x 3 x x  2 5 x 7  x x x 8
x 4 2 x  x x x 6  8 x x x  x 1 x 5

#t11d3p1
Sudoku 16×16 - Hard
https://sudoku-puzzles.net/sudoku-16x16-hard/