Find the decisive blow (difficulty 1125)

87654321 abcdefgh

White to move

Show solution
87654321 abcdefgh
  1. a7–a8=Q key move
  2. c8–d7
  3. a8–b7

What makes it

Best defence still loses — mate in five. A pawn pushes through and promotes. The forced line runs 9 plies.