One of the significant prophecies in A Song of Ice And Fire is centreed around the legendary Azor Ahai, who lived way before our heroes and defeated the darkness that had taken over the world. The prophecy goes that Azor Ahai will be reborn "amidst salt and smoke to wake dragons out of stone" as the Prince Who Was Promised and stand against the darkness again. Azor Ahai followed the religion of the Lord of Light, which is why Melisandre is bent on seeking out the person she believes is the hero reborn and helping them rise to power.
However, we learn in season seven that it might not in fact be the Prince Who Was Promised who defends the world against darkness — it could also be the Princess Who Was Promised. Fans have long thought that Jon and Daenerys are both top contenders to be the hero reborn, as they're both natural leaders who protect others and stand up to evil. Will it be Jon? Will it be Dany? Will it be someone else?
Ominous footnote: The OG Azor Ahai defeated the darkness using an especially powerful sword, which only got its power after he plunged it into the heart of his true love. If either Daenerys or Jon is the hero reborn . . . will one be forced to kill the other in order to be victorious over the army of the dead?