Production is in it's final stages of eighth and final season of Game of Thrones which will be aired sometime in 2019, but today HBO confirmed that they re working on a prequel of the mega hit from feature writer Jane Goldman and author/Game of Thrones co-executive producer George R.R. Martin.
The full summary "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.
not sure when the prequel will air, but in interview earlier , HBO president of programming Casey Bloys said he want their to be a gap between GoT finale and the prequel.
Game of Thrones creators D.B. Weiss and David Benioff will no more work on the franchise.
"There is not going to be anything in any scenario where we have any sort of prequel air for at least a year after that," he told Deadline.
Source: https://www.rollingstone.com/tv/news/game-of-thrones-prequel-pilot-ordered-by-hbo-w521312