Expert 16x16 Sudoku is for solvers who find expert 9x9 grids too short. Swordfish, XY-Wings and chains stretch across a board four times the size, and a single missed candidate can hide the next move for a long time.
Every puzzle is rated 5.0–6.9 by Sudoku Explainer and has one unique solution. Plan on multiple sittings; the board is saved continuously. When you want the same reasoning on a smaller canvas, go back to expert 9x9 Sudoku.