In the closing moments of House of the Dragon, King Viserys names his daughter, Rhaenyra, the heir to the Iron Throne. It's a big moment we expect to be a major part of the series moving forward, but her newfound responsibility also means she is made privy to Aegon the Conqueror's dream, "The Song of Ice and Fire."
So, what does that mean and how does it potentially retcon Game of Thrones? Before we get to that, here is what Viserys tells his heir:
"Our histories, they tell us that Aegon looked across the black water from Dragonstone, saw a rich land ripe for the capture. But ambition alone is not what drove him to conquest. It was a dream. And just as Daenys foresaw the end of Valyria, Aegon foresaw the end of the world of men. It is to begin with a terrible winter, gusting out of the distant north. Aegon saw absolute darkness riding on those winds, and whatever dwells within will destroy the world of the living.
"When this great winter comes, Rhaenyra, all of Westeros must stand against it. And if the world of men is to survive, a Targaryen must be seated on the Iron Throne. A king or a queen, strong enough to unite the entire realm against the cold and the dark. Aegon called his dream 'The Song of Ice and Fire.' This secret has been passed from king to heir since Aegon’s time."
Aegon's dream foresaw the coming of the Night King and the threat his White Walkers posed to all of Westeros. While the prophecy, which clearly suggests only one of fire can defeat ice, is never mentioned in Game of Thrones, we'd be shocked if it isn't a big part of George R. R. Martin's final books when they're released.
As for how it factors into the show, it appears this era's Targaryens will have a noble cause for wanting to remain in power.
The dream makes it clear the only way to stop that threat is for one of them to be on the Iron Throne when the time comes, and Daenerys was, of course. Still, it begs the question of why she was seemingly unaware of this prophecy during her quest for power (which she put aside in an effort to repel the Night King's forces thanks to Jon Snow's - also called Aegon - insistence).
Had she known of it, things might have played out much differently, but seeing as House of the Dragon has established the Night King as the ultimate threat, it's a little strange he and the White Walkers were dealt with in just a few episodes of Game of Thrones' final season.
As for where the retcon comes in, despite the threat of a "great winter" being given little importance by the time all was said and done, we now know it was a prophecy that, regardless of whether later Targaryens knew about it, was always destined to play out. We have to wonder, though, who destroyed it in order to stop the knowledge from being passed down, why, and will that ultimately factor into what happened with the Night King? Perhaps it referred to some other future event?
Remember, Jon Snow/Aegon Targaryen was last seen heading beyond The Wall further North, and with a spinoff series in development, this plot thread could be addressed there as well. One thing we do know for sure is that if this prophecy is going to factor into Martin's novels, his "season 8" could look a whole lot different than what ended up on TV...