Find the decisive blow (difficulty 1155)

12345678 hgfedcba

Black to move

Show solution
12345678 hgfedcba
  1. d4–d3 key move
  2. c2–d3
  3. e1–d1

What makes it

A pawn move and a 1-point investment: mate in five. It is a discovered attack: the moving piece clears the way for the one behind it. 9 half-moves of forced play.