The last hope.
1. f2f4 The most natural move, opening the diagonal for the queen and attacking a7. 1... a8b6 A peculiarity of FreeStyle Chess: it's possible that Black's first move is already clearly better than the alternatives, as is the case here. 2. b1c3 Black has to be careful with the move Nc3-b5, attacking a7 and c7. 2... f7f5 Now White shouldn't play 3.Nb5 as Black can take on a2. 3. a1b3  3... b8a6 Caruana keeps the c7-pawn protected. 4. d2d3  4... c7c6 Forever preventing Nb5, but the position of the knight on b6 is weakened. 5. c1e3 Immediately exploiting the problems with Black's last move. 5... g7g5 The best chance to create counterplay. Caruana undoubtedly had prepared this move in advance. 6. f4g5 Carlsen accepts the pawn sac. 6... e7e5 The idea with the sac is to control the center with the pawns. Next is d7-d5. 7. g2g4 A subtle move to gain access to the e4-square. 7... f5f4 A forced reaction. 8. e3b6 Moving the bishop to d2 or f2 could also be considered. 8... a7b6  9. g1b6 White has two possible alternatives: 9... g8g5  10. b3a5 Carlsen goes after the b7-pawn, but this isn't the best. 10... e8f8  11. h1e4 A difficult and ingenious move. The bishop goes to f5. 11... g5h4 It was better to save the check for later. 12. e1d2  12... h4e7  13. e4f5  13... a6c5  14. c3e4  14... c5e6  15. f5e6  15... e7e6  16. a5b7  16... c8b7  17. b6b7 Now Black has an additional option. 17... d8b8  18. b7c7 Black's position is difficult, but now comes the losing mistake. 18... b8b2  19. d1b1 Carlsen grabs the b-file and the game is essentially over for a player with his magnitude of competence. 19... e6a2  20. b1b2  20... a2b2  21. c7d7  21... b2b4  22. c2c3  22... b4b2  23. d2e1  23... h8g7  24. d7e6  24... g8h8  25. f1f3  25... b2c1  26. e1f2  26... c1h1  27. f3h3 The rook defends and attacks.