Here's the network's official description:

“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. 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."

According to Entertainment Weekly, The Age of Heroes began 10,000 years before the events of Game of Thrones, and storylines could include the construction of the Wall and Winterfell, the generation-long winter known as the Long Night, and the events that led to a war against the white walkers.

Since Game of Thrones is set to end with its eighth season next year, we would not see the prequel, if it's ordered to series, until at least 2020.

This isn't the only Game of Thrones spin-off HBO has planned – the network has five in the works.

Most recently, Entertainment Weekly reported that a spin-off from the Game of Thrones executive producer Bryan Cogman was underway. Details of the others are under wraps, but they are unlikely to include any characters from the present series.

The Game of Thrones showrunners David Benioff and D.B. Weiss, meanwhile, will move on to write and produce a new series of Star Wars films.

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This article originally appeared on Business Insider.