18..fxe6 19. Qg5+ Kf7 20. Qxh5+ Kg7 21. Rh3

It's remarkable how little Black can do that's useful with the tempo at Move 21. It's also a little surprising how few alternatives he has on Moves 19-20. Less surprising is that he has no useful alternatives at Move 18.

Cheers,

The knight avoids Qg5+. This suggests 18.Ne6 fxe6 (18... Q<moves> 19.Qg5+ Kh7 20.Qg7#) 19.Qg5+ Kf(h)7 20.Qxh5+

A) 20... Kg7 21.Rh3

A.1) 21... dxe5 22.Rg3+ Kf6 23.Qg6#.

A.2) 21... Nxe5 22.Rg3+ Kf6 23.dxe5+ dxe5 24.Qg6#.

A.3) 21... Rg8 22.Rg3+ Kf8 23.Qh6+ Kf7 (23... Ke8 24.Rxg8+ Nf8 25.Rxf8+ wins) 24.Qh7+ Ke8 (24... Kf8 25.Rxg8#; 24... Rg7 25.Qxg7+ Ke8 26.Qg6+ Kf8 27.Qg8#) 25.Rxg8+ Nf8 26.Qg7 Kd7 27.exd6 followed by h5, h6, etc. looks winning.

A.4) 21... Rh8 22.Rg3+ Kf8 23.Qxh8+ Kf7 24.Rg7#.

A.5) 21... Rf5(6) 22.Rg3+ Kf8 23.Qh8+ Kf7 24.Rg7#.

A.6) 21... Qe8 22.Rg3+ Qg6 23.Qxg6+ Kh8 24.Qg7#.

B) 20... Kg8 21.Rh3

B.1) 21... Rf5(6) 22.Rg3+ as in A.5.

B.2) 21... Re8 22.Rg3+ Kf8 23.Qh8+ Kf7 24.Q(R)g7#.

Stockfish 10 says that Black should sac its Queen:

18. Ne6

(18. Ne6 (♘g5-e6 f7-f6
♘e6xd8 ♖a8xd8 e5xd6 ♗a6-c4 O-O-O e7-e6 g2-g4 h5xg4 ♖d1-g1 ♔g6-f5 ♕d2-d1 ♖f8-g8 h4-h5 ♗c4-d5 ♖h1-h4 e6-e5 ♖h4xg4 ♖g8-h8 ♖g4-g7 ♔f5-e6 ♖g1-g6 ♖h8-e8 h5-h6 ♔e6xd6 h6-h7 ♖e8-f8 ♖g6-h6 ♔d6-c7 d4xe5 f6xe5 ♕d1-h5 ♖f8xf2 h7-h8♕ ♖f2-f1+ ♔c1-d2 ♖d8xh8) +9.33/31 169)

<18.?>

This is such an attack where you don't really have to think of all specific lines, and it's relatively obvious that Black's lacking of shielding is his downfall.

After <18.Ne6> White poses Black with another piece sacrifice and once again it cannot be declined. The main threat is Qg5+ Qg7# and nothing but fxe6 will also prevent Nxd8. <18....fxe6 19.Qg5+ with 20.Qxh5+> now it doesn't really matter if Black went to f7 or h7, as he's forced to go back to the g-file regardless.

Okay, so I said that not knowing all lines is possible with these kind of positions. Somewhat true, perhaps, but White did put two pieces in: the defense can't go without great return for White.

With 20....Kg8, Black poses himself into more trouble for allowing 21.Qg6+ Kh8 22.Qh6+ Kg8 23.Rh3 with mate: it can never be a good idea to be driven more back than the King needs if he's lacking a proper shield. After Rh3 it's mate in 6.

The key is 20....Kg7. Now White's out of any reasonable tempos and has to continue with 21.Rh3. Now there are very few moves that actually prevent mate:

21....Nf6 22.Rg3+ Ng4 23.Rxg4# doesn't, nor 21....Nxe5 22.Rg3+ Kf6 (Ng4 23.Rxg4+ Kf6 24.Qg6#) 24.dxe5+ dxe5, the latter gives the following 'shield'

click for larger view

But that's not really what a good pawn shield is, 25.Qg6# :>.

The only move that seems to work is <21....Rg8> as it puts a piece on the g-file, and here I made the only hiccup by 'simply' playing 22.Rg3+?! It wins, after 22....Kf8 23.Qh6+ Kf7 24.Qh7+ Ke8 25.Rxg8+ Nf8 26.Qg7 Kd7 27.exd6! and White breaks through in the center just in time, however, after <22.Rf3! +-> it's done way quicker.

Could possibly had been set from here.

White to play.

click for larger view

The Rook lift Rh1-h3 5 or 6 moves later had to be seen when White played 15.Nxh4+