cover-Xtreme Thumbnail of a cover-Xtreme puzzle.

Size:
21x21.
Elements:
1, 2, 3, 4, 5, 6, 7, 8, and 9.
Groups: One hundred forty-three total.
The 36 rows in the upper left, upper right, lower left, and lower right puzzles.
The 36 columns in the upper left, upper right, lower left, and lower right puzzles.
The 36 3x3 squares in the upper left, upper right, lower left, and lower right puzzles.
The 8 diagonals in the upper left, upper right, lower left, and lower right puzzles.
The 9 rows in the center puzzle.
The 9 columns in the center puzzle.
The 9 3x3 squares in the center puzzle.
Instructions:
Precise details on group numbering.
Answer grids:

Puzzle Difficulty levels and Examples

Difficulty LevelLowHighExample
Easiest 2 2 Puzzle   ·   3-steps   ·   All-steps   ·   Solution grid
Easy 3 4 Puzzle   ·   3-steps   ·   All-steps   ·   Solution grid
Medium 5 8 Puzzle   ·   3-steps   ·   All-steps   ·   Solution grid
Difficult 9 14 Puzzle   ·   3-steps   ·   All-steps   ·   Solution grid
Challenging 15 20
Tournament 21 29
TournamentPlus 30 45
Perplexing 46 60