A new prequel series for Game of Thrones is rumored to be in the early stages of production. The fan-led Game of Thrones Wikipedia page reported this "leak" and claimed to have received insider information about the four prequel series that are being considered by HBO.
Though previously reported they were developing five different series, George R.R. Martin has revealed they're down to four. "One of those has been shelved, I am given to understand, and of course Jane's pilot is now moving to film. But that does not mean the others are dead."
Jane Goldman's prequel is set to begin filming in October, but we now might have a second series in the works as well. According to this new report, the series has a working title, Empire of Ash, and it will tell the story of the Doom of Valyria.
If you think back to Season 5, Tyrion Lannister and Ser Jorah Mormont visited Valyria, and encountered Stone Men (men who were suffering from greyscale).
"The hills that split asunder, and the black that ate the skies, the flames that shot so high and hot, that even dragons burned," the poem says. "The city of a thousand years, and all that men had learned, The Doom consumed them all alike, and neither of them turned."
Though it is unclear what exactly caused The Doom, this second prequel will most likely expand on the story. It would also further explain how it connects to the Targaryens. The report says the series "is also the story of the origins of House Targaryen, a minor family of dragon-lords in old Valyria who rose to prominence by fleeing to Westeros just before the Doom—leaving them with the world's only living dragons."
If the report is accurate, it seems like this second prequel series will be telling us a story that's much closer to the "A Song of Ice and Fire" series, a story that's more familiar, and would probably have more dragons.