Find the decisive blow (difficulty 1185)

87654321 abcdefgh

White to move

Show solution
87654321 abcdefgh
  1. d6–d7 key move
  2. f8–f7
  3. d7–d8=N

What makes it

A discovered attack, and the pawn carries it through. The pawn runs in to promote, and that is the point. It calculates 13 plies deep.