Casting calls for HBO's upcoming House of the Dragon, the prequel series to Game of Thrones, have reportedly gone out. Based on George R.R. Martin's Westeros companion book Fire & Blood, the new series takes place 300 years before the start of the original show and takes viewers back to the heyday of House Targaryen.

Given that House of the Dragon is still in the early stages, there's no clear release date set in stone. However, Martin's history of the Targaryens is already being shaped into a series by the showrunners: screenwriter Ryan Condal (Colony) and director Miguel Sapochnik (Game of Thrones). The latter's presence is particularly reassuring, as he was responsible for iconic episodes such as "Hardhome" and "Battle of the Bastards."

Knight Edge Media is reporting that they have seen the official casting call for House of the Dragon, revealing a bit more about what fans can expect from the series. Unsurprisingly, it presents Aegon, Visenya, and Rhaenys Targaryen as leads and further suggests that the series will depict the Targaryen invasion of Westeros and rise to power therein. The full descriptions are as follows:

Aegon Targaryen20s, Caucasian Male. Married his two sisters VISENYA and RHAENYS. Withdrawn, quiet, pious. Harsh with those who defy him. Riding the dragon Balerion. LEAD

Visenya Targaryen20s, Caucasian Female. Older sister and wife of King AEGON TARGARYEN. A voluptuous, stern, serious, and unforgiving woman. Riding the dragon Vhagar. LEAD

Rhaenys Targaryen20s, Caucasian Female. Youngest sister of King AEGON TARGARYEN. Kindhearted, graceful, playful, curious, impulsive, and given to flights of fancy, with a mischievous aspect to her personality. Riding the dragon Meraxes. LEAD

The prequel clearly starts with the three heads of the dragon intact, meaning audiences will probably get to spend much more time with these dragons than they did with Daenerys' children. Not to mention that the incestuous overtones of Game of Thrones will be far more prevalent and accepted in the prequel. Of course, book readers and avid watchers will already be aware of the fate that eventually befalls the various members of House Targaryen and their dragons, but House of the Dragon is likely to fill in blanks which George R.R. Martin's companion books and novellas left to the imagination.

Unfortunately, the same will not be true for places East of Westeros, due to Jane Goldman's The Long Night not moving forward. As thrilling as the dragon-riders of Valyria are, much less is known about the origin of the White Walkers and the people of Essos. The known cast of that series was also more diverse than a family of Targaryens could possibly be - just look at the new casting calls for proof. Hopefully, House of the Dragon can branch out in its storytelling and include more characters outside of Westeros, allowing for a few supporting cast members from Dorne and the Summer Islands, for example.

In the meantime, these reports provide much food for thought if true. Aegon and his sisters were viewed as heroes in the eyes of many Game of Thrones characters, especially Daenerys. But as fans saw with with the young Targaryen herself, one person's hero is another's villain. It'll be interesting to see which parts of the songs and stories remain true for sweet Rhaenys and stern Visenya, and which ones House of the Dragon disproves entirely.