Here's the official logline: A century before the events of Game of Thrones, two unlikely heroes roamed Westeros... a young, naive but courageous knight, Ser Duncan the Tall, and his little squire, Egg. At a time when the Targaryen lineage still holds the Iron Throne and the memory of the last dragon has not yet faded from living memory, great destinies, powerful enemies and dangerous exploits await these unlikely and incomparable friends.

Known as Dunk & Egg, the drama was one of the ideas for prequels in development at HBO after House of the Dragon. The project, which has had multiple writers, emerged as a front-runner for a series order in recent months.
HBO has just started production on the second season of House of the Dragon. A spin-off series about Jon Snow is also in the works.