- "But my ancestors didn't burn or drown with the rest of their race. Twelve years before the Doom, despite the sneers of his rivals, Aenar Targaryen abandoned the capital with his family."
- ―Viserys Targaryen
Aenar Targaryen was a dragonlord of the old Valyrian Freehold, who moved House Targaryen to Dragonstone right before the Doom of Valyria - which resulted in the Targaryens being the only family of dragonlords to survive the destruction of Old Valyria.
The Valyrian Freehold was a vast empire that ruled most of Essos, dominating half of the known world for millennia. About four centuries before the War of the Five Kings, however, the entire Freehold was destroyed in a single day by a cataclysmic volcanic explosion known as the Doom of Valyria. The fires were so hot that even dragons burned alive in mid-air, and the Valyrian Peninsula itself was rent asunder, with large sections falling into the sea, leaving behind a scattered chain of islands in the Smoking Sea.
All of the Valyrian dragons and their ruling aristocracy of Dragonlords were killed in the Doom - all but one family. Twelve years before the Doom, Aenar Targaryen led House Targaryen to settle the distant, western-most outpost of the Freehold: Dragonstone island, off the east coast of Westeros, across the Narrow Sea. The Targaryens therefore survived on Dragonstone while Valyria burned, and when the dust settled, they possessed the world's only remaining dragons.
Some say that Aenar decided to remove his entire household to such a far-flung outpost because his daughter had prophetic dreams about the impending Doom of Valyria. Others who are skeptical of prophecies and magic say that Aenar lost a power struggle with rival dragonlords in the heartland of the Freehold, and fled to Dragonstone in defeat - and that the story about his daughter's prophetic dreams was just a tale that grew up later to try to obscure their shame.
In the books
It is said that Aenar resettled on Dragonstone due to prophetic dreams by his daughter, Daenys the Dreamer, about Valyria's impending destruction. Rival dragonlords laughed at his doomsday preparations, saying he was fleeing from the political rivalries in Valyria - had the Doom not occurred, they would have been right. Many fools and madman throughout history have prophesied apocalyptic destruction, only for nothing to happen. In Aenar's case, either due to pure chance or actual magical prophecy, Valyria was indeed destroyed, leaving the Targaryens with the only surviving dragons.
Nothing has been mentioned about Aenar's spouse, or his activities in the Freehold before removing to Dragonstone. Actually, a brief line says that he moved with "all his wives" to Dragonstone - which seems to imply that he was polygamousy married to at least three women at the same time (which was uncommon but not unheard of in the Freehold). It is said that the Valyrians established an outpost on Dragonstone 200 years before Aenar arrived there (around the year 600 BL), but it is unclear if the Targaryens themselves established the outpost - in which case it was one of several holdings they possessed but not their home castle, which was in Valyria itself - or, if it was colonized by others, and Aenar purchased ownership of it right before he left, specifically in order to hide at an outpost as far away from Valyria as possible. Whatever the case, Aenar sold all of his remaining landholdings in the Valyrian Peninsula when he removed to Dragonstone.
There were around forty major families of Dragonlords in the Valyrian Freehold at its height, who dominated its politics: the Targaryens were one of these aristocratic families, though they were not one of the most powerful ones. The Dragonlords actually didn't use heraldry the way that each noble House in Westeros does: Aenar's descendant Aegon I invented the Targaryen heraldry of a three-headed red dragon when he invaded mainland Westeros (this article still uses these heraldry icons for convenience).
Aenar actually brought five dragons to Dragonstone, one of which was Balerion. All four of the others later died through as-yet unexplained circumstances - probably not fighting each other, as it was later said that the dragon-battles after the Conquest were the first time dragons had clashed since the Doom. While these four dragons died, they left behind eggs, from which hatched Meraxes and Vhagar - and thus the Targaryens possessed three living dragons by the time of Aegon I and the War of Conquest.
Little is known about the pre-Conquest Targaryens, other than that they spent the full century between the Doom and the Conquest largely in isolation, shepherding their resources, as the chaos of the Century of Blood tore apart what was left of the Valyrian Freehold, and the Seven Kingdoms on the mainland of Westeros continued to squabble amongst themselves as always.
Seven generations of Targaryens lived on Dragonstone before the Targaryen Conquest of Westeros: Aenar was the first, while Aegon I and his two sisters were the seventh. The intervening generations have only been briefly sketched out in The World of Ice and Fire sourcebook, in a few scant paragraphs (though the upcoming Fire and Blood: Volume One may fill in more information):
- 1 - Aenar the Exile
- 2 - Gaemon the Glorious and Daenys the Dreamer - Aenar's son and daughter, who married each other according to Valyrian custom, and co-ruled Dragonstone
- 3 - Aegon and Elaena - son and daughter of Gaemon and Daenys, also wed to each other. They were the first generation of Targaryens born on Dragonstone itself. "Aegon the Conqueror" was later known as "Aegon the First" because he was the first king on the Iron Throne - the numbering scheme doesn't retroactively include this Aegon.
- 4 - Maegon and Aerys - Sons of Aegon and Elaena. After Maegon died he was succeeded by his brother Aerys. As with his father Aegon, this Aerys isn't included in the count of kings on the Iron Throne because he predates it, though two later kings were named "Aerys".
- 5 - Aelyx, Baelon, and Daemion - The three sons of Aerys by an as-yet unknown wife. Aelyx was succeeded by Baelon, and Baelon by Daemion. Why these Targaryens were succeeded by their younger brothers, or how they came to die childless, is currently unknown. Daemion had an unknown wife, apparently not a Targaryen - but he also seems to have had an unknown Targaryen sister.
- 6 - Aerion - son of Daemion by an unknown wife. His later descendant "Aerion "Brightflame" Targaryen (older brother of Maester Aemon) was named after him. Aerion's lawful wife was Velaena Velaryon: House Velaryon were vassals of the Targaryens who, while possessing no dragons, had followed them out of Valyria, and settled nearby Driftmark island. The Targaryens intermarried with the Velaryons several times throughout history when they had no siblings to wed, as the Velaryons were also of pure Valyrian bloodline. It is also said that Velaena Velaryon had a Targaryen mother, but she hasn't been named - possibly Daemion's sister (though the exact explanation of what happened hasn't been given yet). With Velaena, Aerion produced Visenya, Aegon I, and Rhaenys (in that order). It was also widely rumored, however, that Orys Baratheon was actually Aerion's bastard son fathered on some unknown woman.
- 7 - Aegon I, Visenya, and Rhaenys - the Conquest generation of House Targaryen, along with their leading general (and rumored bastard half-brother) Orys Baratheon.
Aenar Targaryen is the earliest member of the Targaryen dynasty that has ever been mentioned in the novels. Thus he is the direct ancestor of the all subsequent members of the family, including all of Aegon I's descendants (including Daenerys Targaryen, Jon Snow, Gendry, etc.). Aenar is 20 generations removed from Daenerys - that is, in line of direct descent, starting with Aenar as the first generation, Daenerys herself is the twentieth generation.