The series, based on George R. R. Martin's book Fire & Blood and a prequel to Game of Thrones, features a large ensemble cast including Matt Smith, Emma D'Arcy, Olivia Cooke, Rhys Ifans, Steve Toussaint, Eve Best, Fabien Frankel, Matthew Needham, Sonoya Mizuno, Tom Glynn-Carney, Ewan Mitchell, Harry Collett, Bethany Antonia, Phoebe Campbell, Phia Saban, and Jefferson Hall.
HBO chief Casey Bloys explained the decision in a conversation with Deadline. According to him, the intention was always to follow the history of the Targaryens as documented in the books.
Those familiar with Fire & Blood know how the fate of this house is ultimately sealed. Therefore, there is a natural end to this particular chapter in the history of House Targaryen, and that end will be reached in the fourth season.
