Aegon Targaryen came from Dragonstone, the westernmost holding of a Valyrian dragonlord house that had fled the Doom a century before he was born and, alone among the Freehold's great families, still commanded living dragons when he reached manhood. In 2 BC he landed at the mouth of the Blackwater Rush with fewer than two thousand men, his two sister-wives Visenya and Rhaenys, and three dragons — Balerion, Vhagar, and Meraxes — and by the time the fighting settled, six of the Seven Kingdoms had bent the knee or been broken into bending it. Dorne alone held out, and would go on holding out for centuries after his death.
He built the Iron Throne, by tradition, from the melted swords of the kings he'd unseated — a thousand blades, or so the boast runs, though no one has ever counted them and lived to report an exact figure. He built King's Landing around it, on a stretch of riverbank his ships had used as a landing camp, and ruled the realm he'd made for thirty-seven years, mostly by leaving the local lords to their own laws so long as they paid their taxes and answered his summons.
He rode Balerion, the Black Dread, the largest dragon ever to nest in Westeros and old enough to have flown over Valyria before its fall — and one that would go on flying for another fifty-seven years after him, into his son's and then his grandson's reigns, outliving the empire that bred it by two centuries and the king who first rode it by more than half of one. Aegon himself died in 37 AC, in his sleep by most accounts, and left the throne to a son by Rhaenys — not, as the custom he'd imported from Valyria might have suggested, to either of his sister-wives directly.