'Game of Thrones' Prequel Title Revealed: 'The Long Night'

HBO's Game of Thrones prequel officially has a title, with author George R.R. Martin revealing that the series will be named The Long Night.

Martin — who has penned the series of novels that the franchise is based on, made the announcement Tuesday — revealed the title Tuesday while reacting to the recent casting of Naomi Watts

"Casting is now underway for THE LONG NIGHT, the first of the GAME OF THRONES successor series to ordered to film," he tweeted. "HBO has just announced the first cast member: NAOMI WATTS is coming on board as one of our stars."

Casting is now underway for THE LONG NIGHT, the first of the GAME OF THRONES successor series to ordered to film. HBO has just announced the first cast member: NAOMI WATTS is coming on board as one of our stars. //t.co/n0XkxsJG03 pic.twitter.com/uNF5mjPNte

— George RR Martin (@GRRMspeaking) October 31, 2018

Martin had previously revealed on his blog that his top pick for the sequel's title was The Long Night.

The reveal, possibly an accidental slip, follows on the heels of Tuesday's announcement that actress Naomi Watts had been cast as "a charismatic socialite hiding a dark secret." Currently, Watts is the only actor to be announced for the prequel, though the project is actively casting for numerous roles and also seeking a director.

From writer Jane Goldman and Martin himself, details surrounding the prequel remain a mystery, though HBO did release an official synopsis.

"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," the official description for the series reads. "And only one thing is for sure: From the horrifying secrets of Westeros' history to the true origin of the white walkers, the mysteries of the East, to the Starks of legend…it's not the story we think we know."

Currently, the streaming service has only ordered a pilot episode for The Long Night, though if it is picked up for a full season, it will mark the first time HBO has made a follow-up to one of its series.

The Long Night, set to begin filming sometime in 2019, is not the only prequel series in the works, Martin has revealed, writing on his blog that "there are still a couple of other possible prequels in active development." The specifics of those prequels have not yet been revealed, though it is possible that they will deal with the history of one of the Houses currently on GoT, like the early Targaryen family.

Game of Thrones is set to return for its eighth and final season in spring of 2019.