Find the decisive blow (difficulty 1065)

12345678 hgfedcba

Black to move

Show solution
12345678 hgfedcba
  1. d5–b4 key move
  2. d3–b2
  3. e6–a2

What makes it

Mate in four, bought with 1 points of material given up. The shot starts with the knight. 7 half-moves of forced play.