According to legend, the founder of House Casterly was Corlos, son of Caster, a huntsman who tracked and slew a lion that had been preying on his village's sheep near where Lannisport would one day rise. Sparing the lion's newborn cubs, Corlos won the favor of the old gods, who sent a shaft of sunlight into the beast's den and revealed a vast vein of gold within the mountain. Whether or not the tale is true, Corlos or some other early lord began to mine that gold and fortified the cave against thieves; over generations his descendants delved ever deeper, raising the halls, stairways, and galleries that became Casterly Rock. Though they never claimed a crown, the Casterlys grew into the richest and most powerful house of the First Men in the westerlands.
Their reign ended during the Age of Heroes, when, by legend, the Andal adventurer Lann the Clever won Casterly Rock away from them by guile rather than force, greasing himself with butter to slip through a narrow gap in the rock and driving the Casterlys out with tricks, vermin, or, in the grimmer tellings, smuggled lions that killed the Casterly men before Lann got the women with child. Archmaester Perestan offers a plainer account: that Lann was a household retainer who wed Lord Casterly's daughter and inherited the Rock through her when her father died without a trueborn son. Whichever is true, House Casterly vanished from the earth entirely, and House Lannister has held Casterly Rock ever since.
The completeness of the Casterlys' disappearance became proverbial throughout the Seven Kingdoms, a byword for the price of crossing House Lannister. Centuries later, Ser Jaime Lannister would invoke their fate approvingly when reminded that old wounds never heal: kill the sons along with the father, he told Hoster Blackwood, "ask the Casterlys about that if you doubt me."