Secure the way

Secure the way

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In today's tactical training from Blokh's Combination Motives, I was stumped on the following position and couldn't conceive a candidate move

 

Can you solve?

 

Blokh Diagram 570 Difficulty 6  

What made this difficult for me is that I was lured in by looking at trying to make Nc8 forks work.
 
I didn't appreciate the power of White's two passed a- and c-pawns, coupled with Black's King being cut off from the d-file
 
Particularly instructive is this manouevre Rd8-b8, covering the c8 queening square
 
Lesson Learnt
 
Advanced pawns want to be pushed, put the effort in to finding a way