Find the decisive blow (difficulty 1305)

87654321 abcdefgh

White to move

Show solution
87654321 abcdefgh
  1. c5–c6 key move
  2. c8–c7
  3. b4–b7

What makes it

Mate is forced in seven. A quiet move, soft as anything, and yet there is no reply. It calculates 13 plies deep.