Find the decisive blow (difficulty 1125)

12345678 hgfedcba

Black to move

Show solution
12345678 hgfedcba
  1. g8–a2 key move
  2. b1–a2
  3. g1–c1

What makes it

Mate in four, bought with a 3-point investment. The key move is the bishop's. 7 half-moves of forced play.