Find the decisive blow (difficulty 1530)

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Black to move

Show solution
12345678 hgfedcba
  1. d8–c8 key move
  2. d5–d2
  3. c3–d2

What makes it

The king sets up a forced mate in ten. No check, no capture: just a quiet move, and the defence caves in. It calculates 19 plies deep.