Find the decisive blow (difficulty 1620)

12345678 hgfedcba

Black to move

Show solution
12345678 hgfedcba
  1. e4–d5 key move
  2. e3–g5
  3. d8–c7

What makes it

A queen given up, and the attack wins by force. A quiet move, soft as anything, and yet there is no reply. 29 half-moves of forced play.