The Game of Thrones prequel TV series is slated to begin filming in Ireland by February 2019. While the show doesn't have an official name yet, it's reportedly going by the working title The Long Night and was created by Jane Goldman, the prolific screenwriter whose credits include the fantasy novel adaptations Stardust and Miss Peregrine's Home for Peculiar Children, as well as the Kingsman movies.
Ordered by HBO earlier this year, Goldman's series takes place thousands of years before the events in Game of Thrones and reveals the truth behind the creation of the White Walkers, among other secrets of Westeros history. The prequel is tentatively expected to premiere on HBO in 2020, about a year or so after Game of Thrones airs its eighth and final season (which will run for six episodes).
GWW is reporting that The Long Night pilot may begin filming at Paint Hall/Titanic Studios in Belfast, Northern Ireland (the same shooting stage used for Game of Thrones) as soon as next February. This falls in line with comments that HBO programming president Casey Bloys made on the Television Critics Association tour this past July, where he said that filming on Goldman's prequel series would begin by "early 2019". There have been no casting announcements for the show, at the time of writing.
While there were as many as five different Game of Thrones prequel and/or spinoff TV series in active development at one point, Bloys has since confirmed that Goldman's show is the only one moving forward right now. The Long Night hasn't been officially ordered to series just yet, so HBO could potentially revive one of the other spinoffs it has in the pipeline, in the event that it decides Goldman's prequel isn't how it wants to continue the Game of Thrones TV brand after the original show finishes airing.
HBO doesn't necessarily need to rush Goldman's prequel series down the pipeline to ensure that it has an event series for 2020, either. Given how loaded the network's 2019 slate is already (between Game of Thrones' final season, Big Little Lies season 2, True Detective season 3, and the premiere of Damon Lindelof's Watchmen series), HBO isn't expected to air Westworld season 3 until 2020. Westworld's third season could thus fill the spot that's currently being eyed for Goldman's show, if it comes to that.
Based on Goldman's track record of success, however, the odds are in favor of her Game of Thrones prequel getting a full series order and starting production as planned, early next year. In the meantime, everyone can continue looking forward to finding out who lives, who dies, and who ends up on the Iron Throne when Game of Thrones finally concludes its run (next summer, by the look of things).
The Game of Thrones prequel series (aka. The Long Night) doesn't have an official premiere date yet.