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. Thus far, however, nothing has been confirmed.
The Tales of Dunk and Egg begin 90 years before the War of the Five Kings, at the end of the reign of King Daeron II Targaryen. The realm is still recovering from the First Blackfyre Rebellion thirteen years before, a great civil war that tore the Seven Kingdoms in half. Daeron II's bastard half-brother Daemon Blackfyre had tried to seize the throne in the rebellion, but was killed in the final battle of the war. Several of his sons survived by fleeing into exile, and across the Narrow Sea, House Blackfyre is continually plotting all manner of intrigues against the main branch of the Targaryen dynasty, hoping to gain support for new rebellions.
The Targaryens are still having difficulty maintaining their hold on power, particularly since the last of their dragons died fifty years ago, and the beasts are starting to fade from living memory. There are also lingering tensions from the recent unification of Dorne with the rest of the Seven Kingdoms. The fiercely independent Dornishmen had resisted Targaryen conquest for two centuries, and only finally came under the authority of the Iron Throne through peaceful marriage-alliance: Daeron II married the sister of the Prince of Dorne, and Daeron II's sister married the Prince of Dorne. The terms of the alliance led to the Iron Throne giving House Martell of Dorne preferential treatment, with a strong pro-Martell faction developing around Daeron II at the royal court. This is one of the factors that sparked the Blackfyre Rebellion, as all of the non-Dornish lords who felt that the pro-Martell faction was receiving royal favors which should have been theirs ultimately sided with Daemon Blackfyre. Despite thirteen years of relative peace, relations between the Dornish and the rest of the Seven Kingdoms remain tenuous.
Worse problems are on the horizon, as during the course of Dunk and Egg's first adventures a massive plague known as the Great Spring Sickness sweeps across the Seven Kingdoms (Westeros's equivalent of the Black Death), upsetting what was already a fragile political balance. With plague and drought devastating the Seven Kingdoms, many turn to banditry to survive, and the roads are crawling with outlaw bands. Petty feuds erupt between local lords competing over food and resources. With the realm weakened from within, dangers arise on every side, as the Targaryens' enemies see their vulnerable state as a perfect opportunity to attack. Besides the continuing tensions with the Dornish to the south, the Blackfyres across the eastern sea are attempting to raise new armies in the Free Cities, and to turn lords in Westeros to their side who are upset at the difficulties the Targaryen reign is encountering. To the west, the ironborn led by Dagon Greyjoy are seizing their best chance in generations to return to the Old Way, conducting brazen coastal raids from Bear Island in the North to The Arbor in the south. To the north the wildlings have been united by a new King-Beyond-the-Wall, Raymun Redbeard, and they are preparing to cross over the Wall and sweep through the lands to the south.
Into this backdrop steps Ser Duncan the Tall, a very large squire who grew up as an orphan in the slums of Flea Bottom in King's Landing. After the hedge knight he is in service to dies while heading to a tournament, Dunk takes his gear and enters the tournament as a knight himself. Along the way he strikes up an odd friendship with a young boy who turns out to be Prince Aegon Targaryen, commonly known by his nickname "Egg", a grandson of King Daeron II. Later, however, he runs afoul of Aegon's sadistic and arrogant older brother, Prince Aerion Brighflame, after Aerion accosts a beautiful Dornish woman with whom Duncan becomes smitten. Through a series of many adventures the unlikely duo of "Dunk and Egg" will in the course of years rise to great fame, as Egg grows to become King Aegon V, and Dunk becomes Ser Duncan the Tall, Lord Commander of the Kingsguard.
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 at Summerhall.
On February 10th, 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 on any Westeros-related project in a single year. 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.
Before the beginning of Season 5 of Game of Thrones, HBO programming president Michael Lombardo said in an interview with Entertainment Weekly that the network would eagerly accept the TV series going on for eight to ten seasons, but only if writers Benioff and Weiss felt it served the story instead of dragging it out. This strongly indicates that HBO is interested in continuing to make Westeros-related projects for years to come, such as moving on to a Tales of Dunk and Egg prequel after the main series ends.
The main Game of Thrones TV series has made a few alterations to the overall TV continuity which will affect later seasons of any live-action Tales of Dunk and Egg TV series. In the novels, Aerys II Targaryen - the "Mad King" overthrown in Robert's Rebellion - was Aegon V's grandson. In the novels' the Mad King's father was Jaehaerys II, Aegon V's eldest son. Jaehaerys II had notoriously poor health and died young after only ruling three years, but during that time he ruled well, and effectively dealt with the War of the Ninepenny Kings (the fifth and last Blackfyre Rebellion). Season 1 of Game of Thrones, however, established that the Mad King is actually Egg's son, not his grandson, and the TV producers have confirmed that Jaehaerys II simply does not exist in the TV continuity. Presumably this means that Aegon V's other children (who are characters in the prequel era) have been omitted as well, though the full after-effects of removing an entire generation from the royal family are unknown.
In the novels, the era of Aegon V's reign also saw the slow decline of House Lannister under Gerold's incompetent third son Tytos Lannister, whose hold on power in the Westerlands became so weak that Aegon V actually had to militarily intervene several times to restore order. The very end of the elderly Aegon V's reign, extending into Jaehaerys II's short reign, saw the rise of the young Tywin Lannister, ruthless, cunning, and capable in all of the ways his father was not. First rising to fame as a young warrior in the War of the Ninepenny Kings (not unlike the later Robb Stark), Tywin set about single-handedly rebuilding House Lannister from a laughingstock into the most powerful House in the Seven Kingdoms. During this span of decades the intense rivalry between the Lannisters and their upstart vassals House Reyne played out, culminating in the Reyne Rebellion, during which Tywin wiped out all of his rivals. Aerys II was so impressed with how Tywin crushed his enemies that he named him the new Hand of the King, a position he held for nearly twenty years - until he later betrayed Aerys II to his death in Robert's Rebellion.
The TV continuity removed Jaehaerys II, meaning that the War of the Ninepenny Kings would occur in either the last few years of Aegon V's reign of the first years of Aerys II's reign. Several of the older characters in the main narrative first made their fame in this war: Catelyn Stark's uncle Brynden "The Blackfish" Tully was a commander in the war and hero of half a hundred battles, while ultimately Barristan Selmy personally slew the last Blackfyre pretender, while Tywin and his brothers were first blooded as young warriors in it. If a prequel series is made, there is a good chance it would want to depict the War of the Ninepenny Kings and "rise of Tywin"/Reyne Rebellion storyarcs - even though they climaxed during the reign of Jaehaerys II, who doesn't exist in the TV continuity. However, if they did appear they would chronologically occur so many seasons into a prequel TV series that the narrative would probably just continue after Aegon V died to wrap up loose plot threads, extending directly into Aerys II's reign.