HBO has just announced that Rhys Ifans, Steve Toussaint, Eve Best, and Sonoya Mizuno have joined the cast of their Game of Thrones prequel series, House of the Dragon. The highly anticipated series is based on George R.R. Martin's book Fire & Blood, and it will tell the story of House Targaryen (a.k.a. the House that gave us the Mother of Dragons, Daenerys Targaryen) and take place 300 years before the events of the original groundbreaking series. Production is set to begin in the spring.

Steve Toussaint is set to play Lord Corlys Velaryon, "The Sea Snake" in House of the Dragon. The character is described as the most famed nautical adventurer in the history of Westeros. Lord Corlys built his house into a powerful seat that is even richer than the Lannisters and that claims the largest navy in the world. Rhys Ifans will play Otto Hightower. The Hand of the King, Ser Otto loyally and faithfully serves both his king and his realm. "As the Hand sees it, the greatest threat to the realm is the king's brother, Daemon, and his position as heir to the throne."

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Eve Best will play Princess Rhaenys Velaryon in HBO's House of the Dragon. She is a dragonrider and wife to Lord Corlys Velaryon. "The Queen Who Never Was" was passed over as heir to the throne at the Great Council because the realm favored her cousin, Viserys, simply for being male. Finally, Sonoya Mizuno plays Mysaria. Her character came to Westeros with nothing and has been sold more times than she can recall. "She could have wilted... but instead she rose to become the most trusted - and most unlikely - ally of Prince Daemon Targaryen, the heir to the throne."

HBO has confirmed House of the Dragon will begin filming this April. The 10-episode series, stars Paddy Considine as King Viserys Targaryen, and it will tell the story of the Targaryen civil war that took place about 300 years before Game of Thrones. "Viserys was chosen by the lords of Westeros to succeed the Old King, Jaehaerys Targaryen, at the Great Council at Harrenhal," according to the official cast description.

HBO and HBO Max Chief Content Officer Casey Bloys says the network is already, "talking about areas to develop, which is where this prequel came from." In other words, the network is already looking to expand upon the story that George R.R. Martin created. "We are talking about other areas that make sense as well," he said. "George R.R. Martin's world is so big and what's interesting about it, not only is it big but he's got a lot of road maps in terms of history. So one of the great things about House of the Dragon is that's an established history that leads you to Game of Thrones the show, and there is a lot of little branches. There is a lot of opportunities and stories to tell." Deadline was the first to announce the House of Dragon news.