Find the decisive blow (difficulty 1305)

12345678 hgfedcba

Black to move

Show solution
12345678 hgfedcba
  1. d6–c4 key move
  2. c2–c3
  3. f7–f4

What makes it

Mate is forced in seven. The key move is neither check nor capture — a quiet shot the position cannot answer. It calculates 13 plies deep.