What happened to Old Valyria? The lore House of the Dragon only hints at
Every Targaryen in House of the Dragon is a refugee's descendant. The show mentions Valyria constantly and never shows it, because what happened there predates the series by more than two centuries. Here's the full picture.
Valyria was destroyed by the Doom — a single cataclysmic day, roughly a century before Aegon's Conquest and about 400 years before Game of Thrones. The Fourteen Flames, the volcano chain running through the Valyrian peninsula, erupted at once. The capital was buried, nearly every dragonlord and dragon died, and the peninsula shattered into islands around what became the Smoking Sea.
What Valyria was before that
A civilization of sheepherders who found dragons nesting in the volcanoes and spent the next five thousand years running most of the known world. At its height the Valyrian Freehold controlled Essos from Pentos to Meereen, powered by dragons, Valyrian steel, and blood-and-fire sorcery, and staffed by slaves working the mines beneath the Fourteen Flames.
George R.R. Martin's in-universe history The World of Ice & Fire (2014) describes the day itself in one line:
"Every hill for five hundred miles exploded."
Molten rock a thousand feet in the air, earthquakes leveling temples and towns, lakes boiling or turning to acid, and red clouds raining dragonglass. Ships still refuse to sail within sight of the ruins.
Nobody in-universe agrees on the cause
The maesters have no consensus, which is deliberate on Martin's part. The competing explanations:
- Natural eruption — the mainstream view, and the one most maesters hold.
- Failed spells — Septon Barth's theory that Valyrian magic had been holding the Fourteen Flames in check, and it stopped working.
- The curse of Garin the Great — a Rhoynish prince whose people Valyria destroyed, said to have cursed them with his dying breath.
- Assassinated fire mages — feuding dragonlords killing each other's sorcerers, leaving nobody to keep the volcanoes quiet.
How the Targaryens survived
They left twelve years early. Daenys Targaryen, remembered as Daenys the Dreamer, dreamed of Valyria's destruction. Her father Aenar sold the family 's holdings, packed up his household and five dragons, and relocated to a rock in Blackwater Bay called Dragonstone. The other Forty Families laughed. Twelve years later they were dead, and the Targaryens were the last dragonlords in the world.
What the show actually tells you
Less than you'd think. Viserys explains the family's Valyrian origins to Rhaenyra in the season one premiere, standing under Balerion's skull, and names Daenys. Rhaenyra invokes her again in season three when Helaena's visions come up. The Velaryons get their own Valyrian pedigree — they left for Driftmark even earlier than the Targaryens.
Everything else — the Fourteen Flames, the Smoking Sea, the Forty Families — lives in Martin's supplementary histories rather than on screen. Which is convenient, given HBO has an Aegon's Conquest film in development and a lot of unspent material sitting in those books.