Variety reports that the one-hour show will follow the adventures of Ser Duncan the Tall, and a young Aegon V Targaryen. Set 90 years before the main A Song of Ice and Fire saga, from which GoT was adapted from, the show would likely include events from all three installments from the series, The Hedge Knight, The Sworn Sword, and The Mystery Knight. All three of the novellas have since been published together as A Knight of the Seven Kingdoms.
As of right now, there's no writers, directors, or actors tied to the project. HBO and George R. R. Martin have also yet to announce development on the series officially, but sources say that HBO is eager to produce another Game of Thrones series. If HBO greenlights the project, it will be the second GoT prequel to make it to TV after House of the Dragon, which is expected to premiere sometime in 2022. House of the Dragon promises to detail the lead-up to the Targaryen civil war, which is known as the Dance of Dragons.
Fans have been calling for an adaptation of the Tales of Dunk and Egg saga ever since HBO first announced it was developing various spin-off series as GoT came to an end. It was teased that the novellas were among the contenders to get a TV show adaptation, although Martin later said there were no plans for such a project.
While Tales of Dunk and Egg will be the second GoT prequel to air on TV if HBO greenlights the project, it'll actually be the third prequel to enter production overall. In 2019, HBO announced plans to work on a prequel series starring Naomi Watts, and a pilot was even filmed. However, the project was later shelved for unknown reasons, and post-production on the pilot never wrapped.