The new series is primarily set 170 years before the events of Game of Thrones (not 300, as keeps being erroneously reported) and tells the story of the Dance of Dragons, a huge civil war between two branches of House Targaryen, sparked when the king dies and leaves the Iron Throne to his daughter Rhaenyra, but her much-younger, half-brother Aegon II claims the throne as well, splitting the Seven Kingdoms into two camps. The resulting war is brutal because both sides command dragons in battle.
It sounds like the first season will actually start some time before the war and will show the countdown to the conflict beginning. Announced castmembers so far include Paddy Considine, Olivia Cooke, Emma D'Arcy and Matt Smith.
HBO's chief content officer, Casey Bloys, has also confirmed that HBO is moving forward with development of a second round of script and series ideas, in consultation with A Song of Ice and Fire author George R.R. Martin (the first round resulted in House of the Dragon and a pilot for another series tentatively called The Longest Night, but that project did not proceed further). This second round of discussions includes ideas previously off the table, such as a TV series based on Martin's popular Dunk & Egg short stories. Bloys also confirmed there had been "really embryonic" discussions about an adult, animated series set in the world for streaming service HBO Max.
The newly-announced castmembers are as follows:
Steven Toussaint is a British actor active since the 1990s. He has a history in shows such as Lewis, Berlin Station, Doctros, CSI: Miami, Doctor Who and Pine Gap. In House of the Dragon he is playing the role of Corlys Velaryon, the Sea Snake, the greatest sailor and explorer in the history of the Seven Kingdoms. Having sailed to the remotest corners of the known world, including the distant island of Ib and the mysterious city of Asshai beyond the Jade Sea, he has returned home to the islan dof Driftmark. He is one of the richest and most powerful mean in Westeros and an ally of Prince Daemon Targaryen.
Rhys Ifans is a Welsh actor of long standing. He has appeared in films such as Twin Town, Notting Hill, Enduring Love, The Amazing Spider-Man and Harry Potter and the Deathly Hallows, Part 1. In television he is best-known for playing Mycroft Holmes on Elementary and Hector DeJean on Berlin Station. In House of the Dragon he is playing Lord Otto Hightower, the Hand of the King and the most powerful man in the realm behind only King Viserys. Lord Otto is a political enemy of Prince Daemon, whom he considers a threat to the realm due to his rash temper. Otto's daughter, the Lady Alicent, may be the most eligible would-be bride in the Seven Kingdoms.
Sonoya Mizuno is a Japanese-British actress, best-known for her collaborations with Alex Garland. She starred in his films Ex Machina and Annihilation, and his TV series Devs. Her other roles include La La Land, Beauty and the Beast, Crazy Rich Asians and Maniac. In House of the Dragon she is playing the role of Mysaria, a mysterious woman from Lys who becomes the paramour and confidante of Prince Daemon Targaryen. When war threatens, she reveals skills and talents that will become of great use to her lover.
Eve Best is a British actor best-known for playing Dr. Eleanor O'Hara in Nurse Jackie. She also appeared in The King's Speech as Wallis Simpson and The Challenger Disaster as Sally Ride. In House of the Dragon she is playing Princess Rhaenys Velaryon, the king's cousin who was married to Corlys Velaryon as a political marriage. At one time, Rhaenys was considered an heir to the Iron Throne but was passed over in favour of Viserys simply because of her gender. This has led to Rhaenys being dubbed "the Queen Who Never Was," a cause of some bitterness to her and her husband. Rhaenys is a dragonrider. Her steed, Meleys the Red Queen, is considered one of the most wily and cunning of the Targaryen dragons, and one of the fastest.