HBO has started to move forward on its Game of Thrones prequel.

The network has begun casting for its eagerly anticipated series House of the Dragon, sources confirm to EW.

The project is based on George R.R. Martin's Fire & Blood, a history of House Targaryen set 300 years before the events in GoT.  Ryan Condal (Colony) is the showrunner along with battle-tested GoT veteran Miguel Sapochnik (who helmed episodes such as "The Battle of the Bastards" and "The Winds of Winter"). Sapochnik will also direct the pilot and additional episodes. HBO has committed to 10 episodes of the series.

Game of Thrones Season 8, Episode 4
Credit: Courtesy of HBO

There are no official casting breakdowns yet available, but many are speculating about the potential lead characters based on Martin's book. Fire & Blood covers 150 years and includes the rise and fall of many leaders in Westeros so it's not clear which characters and time period will be the focus of the series. But sources tell EW that the famed Dance of Dragons – the Targaryen Civil War that occasionally referenced in GoT that ripped apart Westeros – will be tackled at some point in the series. What would make sense is if the first season (or two) led up to those events. Some fans have suggested the show could also be an American Horror Story-style anthology series, covering a vastly different time period in each season.

Some key figures in the Targaryen civil war (but not necessarily being cast in the series) include:

-- King Viserys I: The beloved fifth Targaryen king whose reign was peaceful and prosperous until his death.

-- Princess Rhaenyra Targaryen: King Viserys' chosen heir; a dragon rider who grew up expecting to become the first ruling Queen of Westeros.

-- Queen Alicent Hightower: Viserys' ambitious second wife and Rhaenyra's stepmother, who had three children of her own with the king.

-- Aegon II Targaryen: Princess Rhaenyra's younger half brother who challenges Rhaenyra's claim to the throne, igniting a civil war.

From this, it's tempting to speculate on casting. But just because characters are in Martin's book doesn't necessarily mean they will be in the series or be the same as their book descriptions.

House of the Dragon replaces HBO's previously announced prequel project starring Naomi Watts, which filmed a pilot and then was scrapped in 2019 (sometimes the series is referred to as Bloodmoon, which was a code name the production put on signage when filming in Northern Ireland but was not going to be the title of the show). House of the Dragon is planned for 2022.

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