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Now that 'Game of Thrones' is ending, here's what you need to know about the new prequel series

Tom Huddleston Jr.
Jon Snow and Daenerys Targaryen overlooking the Battle of Winterfell on HBO's Game of Thrones.

As they say on the fictional Iron Islands of Westeros, "What is dead may never die": Though 'Game of Thrones' is coming to an end on Sunday night, HBO's massively popular fantasy franchise isn't going anywhere.

HBO has at least three spin-off series in the works based on the books of author George R.R. Martin, led off by a prequel series that has reportedly already started filming and could air sometime in 2020. Martin, who has informally been referring to the prequel as "The Long Night," is an executive producer.

While the network has released precious few details about the upcoming prequel, CNBC Make It tracked down some hints from Martin and his writings to glean information about the plot points and characters of the upcoming series. Here's what you need to know.

When does the prequel take place?

According to HBO, and Martin's personal blog, the events of the prequel show will take place near the end of the Age of Heroes, which was anywhere from 5,000 to 10,000 years before the events of the current "Game of Thrones" series (even Martin says he's not sure of the exact timeline).

The series will include a period referred to in Martin's books as The Long Night (hence his nickname for the prequel), which in Westeros mythology was when humans first defeated the White Walkers and built The Wall, the massive glacial structure along the northern border of the Seven Kingdoms.

Where does it take place?

Just like "Game of Thrones," the prequel will also take place in the fictional land of Westeros. But because the prequel will be set "thousands of years" before the events of the current series, Martin has warned fans they might not recognize the setting.

"Westeros is a very different place. There's no King's Landing. There's no Iron Throne. There are no Targaryens — Valyria has hardly begun to rise yet with its dragons and the great empire that it built," Martin told Entertainment Weekly in November. "We're dealing with a different and older world and hopefully that will be part of the fun of the series."

What will it cover?

HBO isn't saying much, but the network did release this summary: "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. 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, only one thing is for sure: It's not the story we think we know."

But even that short description does offer some important hints as to what the new series will cover. For instance, viewers should finally get to see exactly how the White Walkers first started building the army of the undead that would eventually march south and attack Winterfell in the final season of "Game of Thrones."

Meanwhile, Martin told The Hollywood Reporter that the book "The World of Ice & Fire: The Untold History of Westeros and the Game of Thrones," published in 2014, could contain some clues about the eventual plot of HBO's new prequel series.

Martin's book notes that The Long Night marked the earliest encounters between Westeros humans and the zombie-like White Walkers, and it even describes those White Walkers as riding "monstrous ice spiders" (creatures that could end up cutting into HBO's CGI budget).

In Martin's books, that first invasion by the White Walkers culminated in a fight called "the Battle for the Dawn," where humans banded together with supernatural creatures like giants and the Children of the Forest to fight off their frozen zombie invaders. That means viewers may also get to see how humans defeated the first White Walkers and built The Wall.

While the prequel being set in the distant past means you're probably not going to see any familiar faces from "Game of Thrones," the Age of Heroes did feature distant ancestors of some of the most popular "Game of Thrones" characters, like Brandon the Builder, the legendary founder of House Stark. Those ancestors established the territories and houses that would eventually form the Seven Kingdoms of Westeros.

"From such petty domains arose the mightier kingdoms that came to dominate Westeros in the millenia to come," Martin writes.

Who are the main characters?

In addition to Brandon the Builder (the ancestor of Bran, Arya, Sansa and the rest of the Starks), another important character from the Age of Heroes is called Lann the Clever, who is known is Westeros history as the man who took control of Casterly Rock and founded House Lannister, making him an ancestor of Jaime, Cersei and Tyrion Lannister.

And according to Martin, a central figure of The Long Night is Azor Ahai, a legendary hero who drove off the White Walkers. "Game of Thrones" TV viewers will recognize the name Azor Ahai, because he's frequently mentioned in the series due to the belief that he will be reborn as The Prince That Was Promised to again fight off the darkness of the White Walkers.

Meanwhile, two of the biggest names attached to the prequel series are Academy Award-nominated actresses Miranda Richardson and Naomi Watts. But so far, the only character that HBO has released any hints about is Watts', with the network describing her headlining character only "as a charismatic socialite hiding a dark secret."

How much will it cost HBO?

Francesca Orsi, HBO's senior vice-president of drama, told The Hollywood Reporter last year that when it comes to the prequel, "We are going big."

Early seasons of "Game of Thrones" cost HBO roughly $6 million per episode to produce, but Orsi said that spend "would never fly for what we are trying to do" with the prequel. And HBO's spending on "Game of Thrones" gradually ticked upward over the years, especially with the increased presence of CGI dragons in more recent seasons. Starting in season six, each episode of the series cost around $10 million to produce, according to Entertainment Weekly, though Variety reports that the current final season is estimated to have cost HBO $15 million per episode.

Of course, HBO's big spending worked out well with "Game of Thrones." The smash hit fantasy series is ending an eight-season run that's already garnered a total of 132 Emmy Award nominations (including 38 total wins and three for Outstanding Drama Series) dating back to 2011.

"Game of Thrones" is also the most-watched series in HBO's history, averaging well over 30 million viewers per episode across cable and digital platforms, according to HBO's estimates. Even according to Nielsen's ratings, which don't include digital streams, the show's seventh season averaged 13.7 million viewers per episode.

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