Following the Andal Invasion, the Kingdom of the Rock came to be ruled by House Lannister, who were descended from the Andal invaders and, through the female bloodline, the mythic Lann the Clever who, according to legend, swindled Casterly Rock from the Casterlys during the days of the First Men. The Lannisters ruled as Kings of the Rock until Aegon the Conqueror invaded Westeros. King Loren I Lannister joined forces with Mern IX Gardener, King of the Reach, to wage war against the invaders. However, their forces were obliterated in the Field of Fire. While Mern perished, King Loren submitted to Aegon and the Kingdom of the Rock became the the Westerlands, one of the constituent regions of the unified Seven Kingdoms.
Empires and kingdoms of the known world
Valyrian Freehold · Ghiscari Empire · Kingdom of the Mountain and the Vale · Kingdom of the Isles and the Rivers · Kingdom of the Rock · Kingdom of the Stormlands · Kingdom of the Reach · Principality of Dorne · Kingdom of the Iron Islands · Seven Kingdoms