HBO has officially picked up one of its “Game of Thrones” prequels to pilot, the network announced on Friday.

The untitled project, from George R. R. Martin and “Kingsman” screenwriter Jane Goldman, is set thousands of years before the events of the current “Game of Thrones” series in the era known as the “Age of Heroes.”

According to HBO, the series will chronicle “the world’s descent from the golden Age of Heroes into its darkest hour.”

Also Read: 'Game of Thrones' Spinoffs Are All Prequels With No Current Characters, George RR Martin Says

In the mythology of “Game of Thrones” and Martin’s “A Song of Ice and Fire” novels, the “Age of Heroes” is the period when the First Men settled in Westeros and began to establish what would later become the Seven Kingdoms. The logline for the show promises: “From the horrifying secrets of Westeros’s 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.”

Goldman will write the pilot, based on a story she developed with Martin, and will also serve as showrunner. Vince Gerardis and Daniel Zelman will serve as executive producers. “Game of Thrones” creators D.B. Weiss and David Benioff are not involved with the series.

Goldman’s project was one of five “Game of Thrones” prequels HBO put into development last year, with the others written by “Thrones” alum Bryan Cogman, Max Borenstein (“Kong: Skull Island”), Brian Helgeland (“Legend”) and Carly Wray (“Mad Men,” “The Leftovers”).

Also Read: 'Game of Thrones' Is Going to Kill Off Just About Everyone 'One by One,' HBO Exec Teases

“Game of Thrones,” HBO’s most-watched show in history, is set to end in 2019 with a truncated six-episode season, and the network has been actively searching for a series to replicate its success.

The status of the other prequels is currently unclear, though HBO programming boss Casey Bloys told reporters in January that it’s possible more than one could make its way to air. He also said none of them would premiere until after the conclusion of “Thrones” next year.

“They didn’t start at the same startlines, so we’re at different stages, but I’m very encouraged by what we’ve seen,” said Bloys. “I don’t have a timetable when we’re going to make decisions on anything. I could make between zero and five. Probably more likely around one.”