?? Takes the queen out of the line of fire. Nxc3 followed by Qxb4 was imminent, but this leaves the bishop unprotected after an exchange of knights. Maybe Qe2 or Be2 instead. Rf1 with a threat on Qxf7 checkmate would be tempting, though not sufficient.
16... Nd5xc3
Opens the diagonal to the white king and wins a a piece.
17. bxc3
brings the white queenside rook to play at the expense of the pawn position. Retreating the white light bishop was an option too.
17... Qb6xb5 18. Bd2g5
? This makes the black's advantage in material more pronounced, and the black dark bishop is not an imminent threat.
18... Be7xg5 19. Qg4xg5 h6 20. Qg5h4 g5
chases the queen to make room for the kingside rook
21. Qh4h5 g4
! sets a trap and limits the mobility of the white queen.
22. Qh5xg4
? Qh4 would have disallowed opening the g-file for the rook.
22... Rh8g8 23. Qg4e2 Bb7xg2 24. Qe2xg2
Kg1?? Bf3+ would capture the queen for free.
24... Rg8xg2 25. Kh1xg2 Qb5b2
Prepares or the final assault on the white king. The c-file pawns can't last.