A Song Of Fire and Blood

Aegon of House Targaryen became the King of Westeros after he along with his Sister Wives Rhaenys and Visenya conquered Six of its Seven Kingdoms with the help of their dragons.

The smallest Vhagar who would later by named the 'Queen of All Dragons'. Meraxes the one who was as pale as the moon in the sky. And at last but not the least Balerion 'The Black Dread' who is said to have covered entry towns under the shadow of his wings when he would take flight.

The only Kingdom which was able to resist the conquest of Aegon and his Sister Wives was Dorne. But after years of bloodshed it too was made a part of the Targaryen Dynasty not by conquest 's but by Marriage.

But everything that the Targaryen's have built and conquered is titring at the edge of extinction. After the civil war between the Greens and the Blacks most of the adult dragons died leaving behind eggs and a few hatchlings. But as time went on the dragons grew smaller while the Targaryen's grew weaker whether it be due to the Blackfyre Rebellion or other houses such as the Velaryon and Blackfyre claiming some of the remaining dragons for themselves.

Aerys II Targaryen is the reigning King of the Seven Kingdoms but his rule along with his mind is slowly faltering.