We know a lot about House of the Dragon‘s story. We know all the major figures and events, who will survive, and how it will end. But despite all of that knowledge, the show’s premiere still managed to drop a huge revelation. It’s a monumental piece of lore that reframes the series, Aegon’s Conquest, House Targaryen, and Game of Thrones. Because a dream brought fire and blood came to Westeros to one day lead the fight against the White Walkers.

Once King Viserys decided to name his daughter heir to the Iron Throne, he told Rhaenyra a family secret few Targeryens ever even knew. It’s also a secret never before shared in all of George R.R. Martin’s grand story – the real reason Aegon conquered Westeros.

Paddy Considine as King Viserys Targaryen in House of the Dragon
Ollie Upton/HBO

Before Aegon’s Conquest, House Targaryen stood as the world’s last dragonlords for nearly a century. A minor noble house of Old Valyria, they fled the empire twelve years before its Doom. They only left because Lord Aenar Targaryen’s daughter, known as Daenys the Dreamer, foresaw Valyria’s fall. When the Doom consumed all of the empire and its dragons, House Targaryen was safely on Dragonstone.

But despite possessing the greatest power in the world and residing along Westeros’s eastern coast, House Targaryen barely involved itself in the Realm’s affairs for nearly 100 years, let alone attacked. Aegon didn’t either initially. Before looking west he cared more about what was happening across the Narrow Sea in Essos. That only changed—or so we thought—when King Argilac Durrandon of Storm’s End insulted Aegon. One act of violence from a single old stubborn king led Aegon to conquer the whole continent, not just Argilac’s kingdom.

Until now there’s always been something strange about House Targaryen’s century of inaction followed by Aegon’s overreaction to Argilac. Westeros, rife with in-fighting and millennia old animosity, was ripe for a dragon invasion. And House Targaryen had good reason to do just that. Dragonstone was not the easiest place to live. Now thanks to House of the Dragon we know what changed Aegon’s thinking, and therefore the world forever. Here’s what Viserys told his daughter about Aegon the Conqueror:

Ambition alone was not what drove him to conquest. It was a dream. And just as Daenys saw 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 queen strong enough to unite the 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.

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House Targaryen has a long recorded history of prophetic dreams starting with Daenys. In the novels Daenerys has them. As did a prince named Daeron the Drunken and the Targaryen bastard Daemon II Blackfyre. Now we know Aegon had them as well. He saw the return of the Night King three centuries before the White Walkers breached The Wall. That dream, more than personal ambition, pushed him to take over Westeros. That also helps explain why he let those who bent the knee keep their lands and titles rather than install lords loyal to him. From Aegon’s perspective, he was there to conquer so he could unite Westeros, not destroy or remake it. That was the only way to ensure a Targaryen could lead the charge against the Long Night. His dream fundamentally changes everything we know about Aegon came to Westeros with his sisters.

(Note: Dragons didn’t even exist during the first Long Night. Neither did House Targaryen.)

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But the ramifications of this prophecy, shared only with a king’s heir, changes everything we know about every Targaryen king who followed Aegon. Each carried the burden of a dark secret that involved the very fate of the world. That huge responsibility re-contextualizes every decision any of them ever made. That even includes (without condoning or excusing) Viserys’s decision in House of the Dragon‘s premiere to sacrifice his wife for an heir. Viserys’s obsession with having a son whose claim could not be challenged was at least partly about ensuring a Targaryen sat on the Iron Throne until Aegon’s dream came true.

Aegon’s prophecy now also reframes why Rhaenyra will soon fight a savage civil war to claim the Iron Throne. Her father entrusted her with the most important responsibility in the world. But that secret doomed some of Aegon’s other ancestors, too. It’s the same secret King Daeron the Young Dragon knew of while singularly focused on bringing Dorne under his rule. It’s also a dream Aegon the Unlikely knew about when he died trying to hatch dragon eggs.

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But this prophecy also changes everything we know about Game of Thrones and the events leading up to it. That includes how Daenerys Targaryen never knew about her ancestor’s dream, and why Rhaegar Targaryen and Lyanna Stark’s love led to mankind’s salvation.

One day Rhaegar Targaryen—son of the Mad King, heir to the Iron Throne, and older brother of Daenerys—suddenly decided he had to become a great knight. Some believe he made that decision because he read about the Prince That Was Promised prophecy and thought he was that prince. Now we know it might have been because his father told him of Aegon’s secret. But no matter what made him pick up a sword, eventually he likely believed his future son would be the world’s greatest hero Aegon dreamed of.

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Like Aegon’s dream, that Prince That Was Promised prophecy said a great hero would defeat the darkness and save mankind. But it was not Rhaegar who united the Realm against the White Walkers, nor even a king. It was Rhaegar’s son and rightful heir Jon Snow, whose real name was Aegon Targaryen. Jon Snow never knew about Aegon’s dream, either. But he was the son of Lyanna Stark, and his song was a song of ice (Stark) and fire (Targaryen).

In ways Aegon the Conqueror never could have predicted, his dream came true. A Targaryen united (enough of) the Realm to stand against the Night King and save the living. For nearly three centuries before Game of Thrones, though, that dream also influenced the lives of everyone in Westeros, because it influenced every Targaryen ruler who sat—or, like Rhaenyra, was mean to sit—on the Iron Throne. So much for what we “knew” about House of the Dragon, the Targaryen family, and why dragons came to Westeros.

Mikey Walsh is a staff writer at Nerdist. You can follow him on Twitter at  @burgermike. And also anywhere someone is ranking the Targaryen kings.