HBO's Game of Thrones prequel just added more actors into key roles from George R.R. Martin's fantasy universe.

Game of Thrones prequel House of the Dragon is filling out its core cast, adding four more cast members in key roles.

EW has learned the following are joining the upcoming fantasy series:

Steve Toussaint
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Steve Toussaint (Prince of Persia: The Sands of Time, Judge Dredd) as Lord Corlys Velaryon, "The Sea Snake." The character is described as "Lord of House Velaryon, a Valyrian bloodline as old as House Targaryen. As 'The Sea Snake,' 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
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Rhys Ifans (Notting Hill, Harry Potter and the Deathly Hallows - Part 1) as Otto Hightower - The Hand of the King. The character is described as "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." 

Eve Best
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Eve Best (Nurse Jackie, The King's Speech) as Princess Rhaenys Velaryon - The character is described as "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." 

Sonoya Mizuno
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Sonoya Mizuno (Ex Machina, Crazy Rich Asians) as Mysaria - Her character's backstory is described as, "Mysaria came to Westeros with nothing, 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."

The foursome join:

Olivia Cooke (Ready Player One, Bates Motel) as Alicent Hightower. The character is described as  "the daughter of Otto Hightower, the Hand of the King, and the most comely woman in the Seven Kingdoms.  She was raised in the Red Keep, close to the king and his innermost circle; she possesses both a courtly grace and a keen political acumen."

Matt Smith (The Crown, Doctor Who) as Prince Daemon Targaryen. The character is described as the "younger brother to King Viserys and heir to the throne. A peerless warrior and a dragonrider, Daemon possesses the true blood of the dragon.  But it is said that whenever a Targaryen is born, the gods toss a coin in the air…" Smith has been in talks for the role for a couple of months and the production's interest in the actor was first reported as a rumor by Winter is Coming.

Emma D'Arcy (Truth Seekers) as Princess Rhaenyra Targaryen. The character is described as "the king's first-born child, she is of pure Valyrian blood, and she is a dragonrider.  Many would say that Rhaenyra was born with everything… but she was not born a man." Rhaenyra is considered a major rock star role within the show.

Paddy Considine (Peaky Blinders) as King Viserys I Targaryen. "King Viserys Targaryen is chosen by the lords of Westeros to succeed the Old King, Jaehaerys Targaryen, at the Great Council at Harrenhal. A warm, kind, and decent man, Viserys only wishes to carry forward his grandfather's legacy. But good men do not necessarily make for great kings.")

House of the Dragon chronicles a civil war among House Targaryen in Westeros and is based on George R.R. Martin's book Fire & Blood. The series is expected to begin shooting in April and then air in 2022.

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