Oops. Apparently author George R.R. Martin stuck his foot in it with recent comments about the Game of Thrones prequel, and now Clarification Is Coming. In his blog, GRRM had mentioned the new project being titled The Long Night. But he sent that raven too soon, and returned to his blog after getting a wrist-slap from HBO:
Oh, I should also mention... HBO has informed me that the Jane Goldman pilot is not (yet) titled The Long Night. That's is certainly the title I prefer, but for the moment the pilot is still officially UNTITLED. So... mea culpa, mea culpa, mea culpa. Elsewise, the pilot is coming along well, with casting falling into place. I could tell you more, but I am not supposed to. We also have a couple of other successor shows still in development, but I cannot tell you about those either. Sorry. And some exciting things are happening with the Wild Cards television series, but I can't talk about those. Nor can I say much about WHO FEARS DEATH, based on Nnedi Okorafor's wonderful novel. Hush hush, it's Hollywood, they like news to come through officially approved channels out there.
Since Game of Thrones is based on the author's A Song of Ice and Fire novels, he should be allowed to reveal whatever he wants. And if he wants the title of this first spinoff series to be The Long Night, then maybe let that be it. This prequel is reportedly set thousands of years before the events of Game of Thrones, and "The Long Night" is what the people of Westeros called a (literally) dark time during the Age of Heroes when a great winter lasted for many years.
George R.R. Martin prefers the term "successor shows" to spinoffs, but we know several spinoffs -- all prequels to Game of Thrones -- are in the works at HBO. So far, only one of them has been ordered to pilot. As the author mentioned, it's being led by co-writer and showrunner Jane Goldman. Naomi Watts and Josh Whitehouse were recently cast for it, and it sounds like GRRM knows more at this point, but just can't say it. (He also hinted to more info on his Wild Cards series.)
The author has given some hints on the other potential spinoffs, ruling out Robert's Rebellion or anything on Dunk and Egg. We'll have to stay tuned for more details on this still officially untitled prequel series, plus the others that may or may not move forward to get pilots of their own.
Meanwhile, Game of Thrones Season 8 has wrapped filming, and we recently got new details on how the final season will begin. Those six episodes are meant to premiere in the first half of 2019 on HBO. We know the prequels won't arrive on HBO until after the final season airs, so it will be many long nights before we see The Long Night -- or whatever it's ultimately called.