Tales of Dunk and Egg

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, who though a boy at the start of the series will eventually become King Aegon V.

Story
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 of the Tragedy of Summerhall.

Potential adaptation
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.