While Daenerys has always been a prominent character on Game of Thrones, in truth, we've seen little of the rest of the Targaryen clan. We've seen the Mad King, Aerys II, in brief flashbacks, ranting about burning them all. We saw eldest brother Rhaegar in profile as he married Lyanna Stark. In Season 1, we saw Viserys threaten to release the dragon until Khal Drogo poured a molten gold "crown" over his head. Mostly, we've been told that Daenerys is the sole member of a family that has rightful claim to the Iron Throne—or so she thinks.
But if you look to the books by George R. R. Martin, as well as the accompanying companion pieces that go into the history of the Seven Kingdoms, fans are treated to a much more detailed look at the history of House Targaryen. So, let's learn everything we need to know about the Targaryens, how they came to be the most powerful dynasty in Westeros, and why they are struggling to regain power today.
House Targaryen was just one of 40 houses that presided over the Valyrian Freehold, an empire that spanned most of the eastern continent, Essos. These houses were comprised of noble families called dragonlords: people who could control and ride dragons either through sorcery or with whips and horns. The Targaryens were not the most powerful of these 40 families, but they were the only house to survive the Doom of Valyria.
Daenys Targaryen (presumably Daenerys's namesake) had a prophetic vision of a great cataclysm destroying Valyria. This was enough to convince her father, Lord Aenar Targaryen, to sell his estates and move his family and his five dragons to Dragonstone—the same island off the eastern coast of Westeros where Dany set up shop in Season 7.
Twelve years after the Targaryen exodus to Dragonstone, the Doom of Valyria wiped out the most of the dragonlord families and all of their dragons. So, the Targaryens were correct to have evacuated, and they were left as the only Valryian family with dragons.
The Doom of Valyria (AKA "the Doom") was a natural—perhaps magical—disaster that destroyed the Valyrian Freehold. In addition to catastrophic physical damage, the event also marked the end of all history, magic, and knowledge documented by the 40 families who lived in Valyria.
On the day the Doom occurred, every hill in Valyria erupted, launching smoke and fire into the air. This fire was so powerful, it killed all the local dragons; this is one of the reasons dragons had died out by the beginning of Game of Thrones. Earthquakes destroyed buildings, lakes boiled and turned acidic, and the Valyrian volcanoes ejected molten rock into the air. All this destruction lead to the deaths of nearly every dragonlord, leaving the Targaryens (who'd had the good sense to escape) the only remaining military force in the Seven Kingdoms with access to dragons.
After most of the dragons in Valyria were killed in the Doom, the Targaryens were among a very few families to still own any mountable dragons. During their time on Dragonstone, the Targaryens owned five dragons, but four of those died in the century that followed the Doom. Out of the original five dragons that migrated from Valyria with the family, only one, Balerion the Black Dread, survived.
However, during this time, the Targaryens were able to hatch two more dragons, Vhagar and Meraxes. Vhagar's breath was so hot, it was said he could melt stones and steel alike. Meraxes was the larger of the two dragons, large enough to swallow a horse in one bite.
Aegon I Targaryen, AKA Aegon the Conqueror, was born on Dragonstone after the Doom, roughly halfway through the Century of Blood, a time of brutal unrest in Essos. Aegon was the middle child in his family and the only son. He did what all Targaryens are wont to do: married within the family. However, Aegon didn’t just marry one sister—he married both. His sisters were Visenya and Rhaenys, and they were crucial to Aegon's ambitious plans.
Once the Century of Blood came to an end, Aegon I's interest in Westeros grew, and he became incredibly invested in the idea of uniting the Seven Kingdoms. He didn’t believe the land of the continent should be divided up, but that it should all be one realm. And so he set out to achieve this lofty goal.