You’re about to hear a lot more about the Age of Heroes, a period predating the events of Game of Thrones by thousands of years.
HBO has ordered a pilot for one of the four Game of Thrones prequels in development, with George R. R. Martin noting that more could be greenlit: “Everything I am told indicates that we could film at least one more pilot, and maybe more than one, in the years to come,” Martin wrote on his blog. “We do have an entire world and tens of thousands of years of history to play with, after all.”
HBO’s official logline for the new series reads: “Taking place thousands of years before the events of Game of Thrones, the series chronicles the world’s descent from the golden Age of Heroes into its darkest hour. And only one thing is for sure: from the horrifying secrets of Westeros’s history to the true origin of the white walkers, the mysteries of the East to the Starks of legend… it’s not the story we think we know.”
Source: IGN.com Everything You Need to Know About Game of Thrones' Age of Heroes