Find the decisive blow (difficulty 1530)

12345678 hgfedcba

Black to move

Show solution
12345678 hgfedcba
  1. b8–f8 key move
  2. f3–g2
  3. h3–g2

What makes it

Best defence still loses — mate in 15. The key move is the queen's. It calculates 29 plies deep.