Find the decisive blow (difficulty 1620)

87654321 abcdefgh

White to move

Show solution
87654321 abcdefgh
  1. b6–d7 key move
  2. d8–d7
  3. h1–d5

What makes it

Mate in 12, bought with 3 points of material given up. The shot starts with the knight. 23 half-moves of forced play.