Find the decisive blow (difficulty 1125)

87654321 abcdefgh

White to move

Show solution
87654321 abcdefgh
  1. a5–c6 key move
  2. a7–a8
  3. e2–a6

What makes it

A knight move forces mate in five. A fork: two or three pieces fall under attack together. 9 half-moves of forced play.