HBO's Game of Thrones prequel series is set to begin filming in June, and we now know it will be shooting under the working title Bloodmoon. The show doesn't have an official title yet, though it's often unofficially referred to as The Long Night since it's set during that period of Game of Thrones history - about 8,000 years before the War of the Five Kings.
The Long Night's cast is a mix of established actors and newer faces, with the more well-known names including Naomi Watts (King Kong), Miranda Richardson (Churchill), John Simm (Doctor Who), and Jamie Campbell Bower (Fantastic Beasts: The Crimes of Grindelwald). The principal cast also includes Josh Whitehouse (Poldark), Georgia Henley (The Chronicles of Narnia), Alex Sharp (The Hustle), and Naomi Ackie (Lady Macbeth). A Song of Ice and Fire author George R.R. Martin came up with the story for the show, and Jane Goldman (Kingsman) will serve as showrunner.
In the March issue of Production Weekly (via ComicBook), the spinoff is listed as Untitled Game of Thrones Prequel (w/t Bloodmoon). The working title is thematically appropriate for the world of Game of Thrones, in which blood has played a pivotal role (in more ways than one), but fans shouldn't try to extrapolate too many clues from the phrase 'Bloodmoon'. Working titles are designed to be innocuous, in particular to avoid drawing attention to the production, and so usually have nothing to do with the plot of the movie or TV show in question. For example, Spider-Man: Homecoming was filmed under the working title Summer of George - a reference to an episode of Seinfeld.
Since the prequel show is set so far in the past, we shouldn't expect to see any Game of Thrones characters reprising their roles (with the possible exception of the Night King). The show will predate the Iron Throne and the reign of the Targaryens by thousands of years, and we can expect to see a very different landscape of Westeros and Essos. According to the official synopsis, the show will "chronicle the world's descent from the golden Age of Heroes into its darkest hour" - specifically the Long Night, when the White Walkers moved south and spread terror and darkness. It also promises to reveal "the true origin of the White Walkers, the mysteries of the East [and] the Starks of legend."
Anticipation is naturally high for the new series, now that Game of Thrones has come to an end, but although filming begins soon, fans will likely have to wait until 2021 to see the show. Hopefully we'll get an official title some time before then, so we don't have to spend the next two years talking about 'Bloodmoon'.