A series adaptation of Tales of Dunk and Egg, a prequel to the events of Game of Thrones, is in early development at HBO.
The show would be based on the series of novellas by George R.R. Martin, which follows Ser Duncan the Tall (Dunk) and a young Aegon V Targaryen (Egg) 90 years before the events of A Song of Ice and Fire.
To date, Martin has three novellas in the Tales of Dunk and Egg series: The Hedge Knight, The Sworn Sword, and The Mystery Knight. Since being published separately they have been collected and published together as A Knight of the Seven Kingdoms in 2015.
If the project makes it to series it will be the second Game of Thrones prequel to make it to screen. Currently, HBO is working on the series House of Dragons which is scheduled for a 2022 debut. The show will feature the buildup of the Targaryen civil war in Westeros, known as the Dance of Dragons.
The project is still in early development and no writer or talent has been attached to the project.