Given the details of the casting, and the specific characters that will play key roles in the series, the overall trajectory of the series becomes a little clearer. We know that the core of the story will revolve around Viserys and his attempt to rule Westeros after the death of King Jaehaerys Targaryen. Even though Viserys is a good man, as Game of Thrones has proven time and time again, being good is not always enough to be a successful leader.
House of the Dragon is set to be released in 2022, and despite the fact that few concrete details are known about its story, there is enough ancillary A Song of Ice and Fire material to get a pretty good idea of where the show is heading. Based on George R.R. Martin’s novella The Princess and the Queen, we know that Viserys’ rule sows the seeds for a Targaryen civil war, and, presumably, the show will actually explore what happened during this time that led to the destructive conflict. The casting of Smith, Cooke, and D’Arcy as those specific characters (who all play key roles in The Princess and the Queen) hints at House of the Dragon exploring the beginnings of a conflict within the Targaryen family.
Source: Game of Thrones
Originally from https://screenrant.com/house-dragon-casts-matt-smith-daemon-targaryen/