A weapon that would bring the dawn, after careful forging of a plan, at the cost of Dany's life. Perhaps Dany, and Drogon will sacrifice themselves to destroy the White Walkers, thus fulfilling the final part of the prophecy. The final sword was Lightbringer, that brought the dawn, at the cost of the true love of Azor Ahai, Nyssa Nyssa. Perhaps this means Rhaegal will die, either in the battle up north, due to the actions of the lion (Cersei not having their backs), or in a battle at King's Landing, in revenge for the betrayal, thus killed, while being plunged into the heart of the lion. With the end of season 7, we see that Cersei intends to betray the alliance, an alliance that was carefully orchestrated, or forged, but not without fault, mind you. The second sword was lost, in the heart of the lion. This would still be a loss of a dragon to water, due to poor planning/foresight. Furthermore, Linda mused on her YouTube channel, that she believes Euron may take Viseryon with the dragon binder.

The plan to get the wight was a poorly forged one, like the sword. The first was tempered for a short time, and quenched in water. What if, this story is in fact a prophecy about the dragons? The last of which was of course Lightbringer that brought the dawn. We know Azor Ahai made 3 swords, one he quenched in water, another he tempered longer, and quenched in the heart of a lion, a third, in the heart of Nyssa Nyssa. It's a little off the wall, and might amount to nothing, b ut I'll present it anyway, for discussion. I was recently thinking about the Azor Ahai story, and the three swords he forged, along with the death of Viseryon in the show, and Linda's musings on how that might tie in to Euron in the books, when I saw a YouTube video (can't remember the title unfortunately) that made it all click. I'll start by saying I'm typing this on mobile, so apologies for any typos.

Dawn of the dragons mobile forum