George RR Martin has been on the promotional trail for 'Fire & Blood', the in-universe encyclopedia about the Targaryen dynasty, and dropped some tidbits about the upcoming prequel series, tentatively titled 'The Long Night'.
"Westeros is a very different place," he explained. "There’s no King’s Landing. There’s no Iron Throne. There are no Targaryens — Valyria has hardly begun to rise yet with its dragons and the great empire that it built. We’re dealing with a different and older world and hopefully, that will be part of the fun of the series."
So far, Naomi Watts is the first casting announced for the series and aside from a vague plot synopsis, there's very little information to go on. The synopsis states that the series "chronicles the world’s descent from the golden Age of Heroes into its darkest hour" and promises to reveal "the horrifying secrets of Westeros’ history to the true origin."
As it stands, the series has not yet been greenlit and a pilot episode is still be made so 'The Long Night' - if it ends up being called that - most likely won't hit our screens until at least 2020.