After 1....Nc3, Black is in a predicament. If the pawn takes the knight, the queen takes a3 and mates. If the queen must be saved and the knight recaptured after it takes the rook, then the queen moves to e1 or f3. But then the knight moves to a2 and mates. So, pawn takes bishop which leaves Black with two pieces for the queen after the knight captures it.
Correction
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Nice puzzle.