Find the decisive blow (difficulty 1680)

12345678 hgfedcba

Black to move

Show solution
12345678 hgfedcba
  1. e6–e7 key move
  2. f7–e7
  3. d7–e7

What makes it

A rook move and a 5-point investment: mate in 13. A quiet move, soft as anything, and yet there is no reply. It calculates 25 plies deep.