The history of House Targaryen stretches back several centuries to ancient Valyria. After surviving a cataclysmic collapse of the existing order in Essos, House Targaryen was left as the last house of dragon-riders.
Following a period of war in Essos, Aegon Targaryen (later Aegon I) set his sights on conquering the lands to the west: Westeros. He intended to unite the Seven Kingdoms under one rule, and within a few years, he had conquered six of the seven kingdoms of Westeros. The only holdout was Dorne, which eventually joined the fold in a formal capacity but retained more autonomy.
Aegon I founded the Targaryen dynasty that would rule over Westeros for nearly three centuries. The royal house faced a number of uprisings, both political and religious, but held onto power through the reigns of both cruel and wise kings. Several generations down the line, the fifth Targaryen king to sit on the Iron Throne was Viserys I, the grandson of Jaehaerys I, the Conciliator.