HBO already has one massive fantasy series in Game of Thrones, but now they’re adding another one. Though it’s not theirs exclusively.

It’s being reported that HBO has reached a deal to co-produce the BBC series His Dark Materials, which is based on author Philip Pullman‘s book trilogy of the same name. The report says that the series is likely one of the biggest and most expensive British series ever made.

HBO will produce His Dark Materials along with BBC, Warner Brothers’ film production subsidiary New Line Cinema, and UK production company Bad Wolf, which HBO owns a minority stake in.

New Line being involved makes sense. They and Warner Bros. were behind the 2007 movie The Golden Compass, which was based on Northern Lights, the first book in Pullman’s “Dark Materials” trilogy. The other two books are The Subtle Knife and The Amber Spyglass. There will also be more material to work with if they wish to use it, such as the “Book of Dust” companion trilogy Pullman is currently working on.

Here’s a description of the first season of the show:

The first season of His Dark Materials follows Lyra (Dafne Keen), a seemingly ordinary but brave young woman from another world. Lyra’s search for a kidnapped friend uncovers a sinister plot involving stolen children, and becomes a quest to understand a mysterious phenomenon called Dust. As she journeys through the worlds, including our own, Lyra meets a determined and courageous boy called Will. Together they encounter extraordinary beings and dangerous secrets, with the fate of both the living””and the dead”” in their hands.

The cast for His Dark Materials includes Dafne Keen, James McAvoy, Ruth Wilson, Lin-Manuel Miranda, Anne-Marie Duff, Clarke Peters, Ariyon Bakare, Will Keen, Ian Gelder, Georgina Campbell, Lucian Msamati, James Cosmo, Ruta Gedmintas, Mat Fraser, Geoff Bell, Simon Manyonda, Lewin Lloyd, Daniel Frogson, and Tyler Howitt.

You can find a description of who each actor is playing over at Deadline, who shared the news and the description above.

Writing on the series is Jack Thorne, whose credits include Wonder and the in-the-works adaptation of Neil Gaiman’s The Sandman. Directing the first two episodes is Oscar winner Tom Hooper (The King’s Speech, Les Misérables (2012), The Danish Girl, and the HBO miniseries John Adams). Also directing episodes is Otto Bathurst (Robin Hood, Peaky Blinders), Jamie Childs (Doctor Who, Vera), Euros Lyn (Doctor Who, Sherlock, Daredevil), and Dawn Shadforth (Trust).

While His Dark Materials is a BBC production, HBO’s deal to co-produce actually includes the worldwide rights to the series outside of the UK.

No release date has been announced yet, but the show began filming earlier this month.

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