The Tales of Dunk and Egg are a series of prequel novellas written by George R.R. Martin, which begin 90 years before the events of the A Song of Ice and Fire novels (on which the TV series Game of Thrones is based). They follow the adventures of Ser Duncan the Tall and his squire, Prince Aegon "Egg" Targaryen - the younger brother of Maester Aemon. Though Egg is a boy at the start of the series he will eventually grow up to become King Aegon V, the Unlikely.
The Tales of Dunk and Egg may potentially be made into a live-action project which will be a prequel to the main Game of Thrones TV series.
Martin has said that the novellas will include time-jumps so that they will ultimately cover the rest of the lives of the two main characters, stretching across a period of 50 years, until both of them died in the Tragedy of Summerhall. This will bridge the timeline between the two main narrative eras: Ser Barristan Selmy was knighted at the age of sixteen by King Aegon V after unhorsing Ser Duncan in a joust. Tywin Lannister's grandfather Gerold Lannister is a character in the early novellas, but young Tywin was a boy when Aegon V died. Prince Rhaegar Targaryen was also born the very same night as the Tragedy of Summerhall.
On 10 February 2013, George R.R. Martin confirmed that he had been in discussion with HBO over a possible adaptation of the Tales of Dunk and Egg short stories, though in what format (TV movies, theatrical movie, or a spin-off TV series) was not made clear.
The HBO production team from the Game of Thrones TV series has said that they dismiss out of hand the possibility of starting any prequel project before the main series if finished, given the massive amount of work involved on it. They physically cannot produce more than ten episodes a season. While potential future prequel projects are being discussed, this is mostly legal negotiation at this point, and no pre-production work will be started until after the final season of Game of Thrones.