Reign Toby Regbo

The CW

The population of the "Game of Thrones" prequel series is growing.

Eight up-and-coming actors have been cast as series regulars in HBO's as-yet-untitled drama, which is set thousands of years before the events of the flagship.

Joining already cast stars Naomi Watts and Josh Witehouse are: Naomi Ackie ("Star Wars: Episode IX"), Denise Gough (Tony-nominated for "Angels in America"), Jamie Campbell Bower ("Twilight"), Sheila Atim ("Harlots"), Ivanno Jeremiah ("Black Mirror"), Georgie Henley ("The Chronicles of Narnia"), Alex Sharp ("To the Bone") and Toby Regbo ("Reign").

The story chronicles the world's descent from the golden Age of Heroes into its darkest hour. And as HBO's official synopsis teases, ""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."

Watts plays a charismatic socialite hiding a dark secret, but the other character descriptions have not been revealed.

The pilot is based on a story by writer Jane Goldman and franchise author George R.R. Martin, with the script by Goldman. SJ Clarkson, who will be the first woman to direct a "Star Trek" movie, will helm the pilot.

"Game of Thrones" is set to conclude after an eighth and final season starting this April. It's unknown when the prequel, should it be picked up after the pilot, would air.