The Dragonpit Summit was a parley of the major monarchs of Westeros in King's Landing to discuss the future of the Seven Kingdoms after Daenerys Targaryen's war for Westeros comes to a stalemate and an armistice proposed by Lord Tyrion Lannister is agreed upon.
Cersei Lannister holds the crown after the collapse of the Baratheon dynasty, claiming it by right of conquest. Daenerys Targaryen claims the Iron Throne on the basis that it belongs to the once royal House Targaryen, which was overthrown by House Baratheon in the war known as Robert's Rebellion.
Jon Snow, who is said to be a bastard of House Stark, rules the Kingdom of the North as the King in the North, though he has become an ally of Daenerys Targaryen. He seeks to use the gathering to provide proof of the existence of the Night King and the White Walkers through a captured wight and rally the Iron Throne to his cause.
Euron Greyjoy sits on the Salt Throne, the throne of the Kingdom of the Iron Islands, as the King of the Iron Islands, though he has since allied with the now royal House Lannister. His hold of the Salt Throne is disputed by his niece and nephew, Yara and Theon, the former who is now a prisoner of Euron, and both of whom are allies of Daenerys Targaryen.
Robert's Rebellion saw the ousting of the Targaryen dynasty that had ruled the unified Seven Kingdoms of Westeros since the Targaryen Conquest nearly three centuries prior. However, civil war broke out in the form of the War of the Five Kings after the death of Robert Baratheon as three kings claimed the Iron Throne while two others attempted to secede from the Seven Kingdoms.
The War of Five Kings greatly weakened the Baratheon dynasty, which had become a puppet of House Lannister after Robert's death through the reigns of his alleged sons, Joffrey and Tommen, who were actually bastards born of incest between Robert's wife, Cersei Lannister, and her twin brother, Jaime Lannister. They passed off Joffrey, Tommen, and Myrcella as Robert's children to keep them alive.
Eventually, Cersei herself ascended to the Iron Throne after destroying her gathered enemies - the Faith Militant and House Tyrell - at the Great Sept of Baelor. The event inadvertently killed her last surviving child, Tommen, thus ending the Baratheon dynasty and leaving a power vacuum that Cersei took advantage of, claiming the crown for herself and beginning the Lannister dynasty. However, war had not ended in Westeros: Daenerys Targaryen gained allies in the enemies that Cersei had made in the Reach and Dorne, two of the Seven Kingdoms, and began her war for Westeros to restore her family dynasty.
Euron Greyjoy had become King of the Iron Islands after murdering his brother, Balon Greyjoy. After Balon's children, Yara and Theon Greyjoy, beat him to forming a pact with Daenerys, he instead allied with Cersei.
Jon Snow, former Lord Commander of the Night's Watch, had become King in the North after defeating House Bolton at the Battle of the Bastards, avenging the Red Wedding which had seen the deaths of Robb Stark and many of his Northern and Riverland bannermen. Jon Snow's time in the Night's Watch revealed to him that an ancient threat to all of humanity had returned after 8,000 years: the White Walkers. The Night King led the White Walkers on a crusade beyond the Wall to massacre the Free Folk and reanimate their corpses as wights, adding them to his army of the dead. Jon mobilized the North and the Vale to prepare for the coming Great War against the White Walkers and the return of the Long Night, a dark and terrible winter that lasted for over a generation when the White Walkers first descended upon the world.
However, because the White Walkers had been gone for thousands of years, they had faded into legend: most in northern Westeros had believed that they were extinct (prior to Jon Snow's coronation as King in the North), while everyone else believed that the White Walkers never existed in the first place and were simply tales and folk legends. With the Seven Kingdoms preoccupied with their own civil wars, Jon Snow sought to take advantage of the rare summit by gathering proof of the return of the White Walkers: by capturing a wight at the Wight Hunt beyond the Wall. With a band of his followers, Jon Snow and his men were able to capture a wight beyond the Wall near Eastwatch-by-the-Sea, though they lost a few men as well as one of Daenerys Targaryen's dragons, Viserion, who had, unbeknownst to them, been resurrected by the Night King.
Prior to the parley, each side positioned their men to retaliate if one of the belligerents was to use the gathering to slaughter the leaders of the opposing side. The remnants of the Lannister army and the City Watch guarded the walls of King's Landing while the Iron Fleet protected the city's harbor. However, as the Targaryen army had already landed and was supported by dragonfire, had a battle occurred the outnumbered Lannisters would have been slaughtered. Daenerys's forces, both the Unsullied and the Dothraki, were massed near the capital's fortifications under the command of Grey Worm.
Bronn welcomes Jon, Tyrion and Daenerys' advisors and leads them to the Dragonpit. Bronn invites Pod to go somewhere else and have a drink. While they are walking away, Jon, Brienne, Jorah and Tyrion become worried, looking around nervously, as if they suspect of a trap. The Hound also expresses his concern. Then Cersei, Jaime, Euron and their followers arrive, with Cersei expressing irritation that Daenerys has not come with the others and is keeping them waiting.
After Daenerys arrives dramatically on the back of Drogon, the meeting begins. However, as Tyrion speaks to open the parley, Euron suddenly interrupts and mocks Theon by reiterating that he has Yara and will kill her, while also mocking Tyrion; when Tyrion and Theon respond to Euron's taunts with their own, the elder Greyjoy angrily retorts that dwarves are killed at birth in the Iron Islands. A furious Jaime orders Euron to sit down, which Cersei reiterates when Euron dismisses Jaime's warning. A subdued Euron returns to his seat and the meeting resumes. Tyrion explains that they have suffered at each other's hands, and that they are capable of waging war without meeting face to face, but that is not what they are here for. Cersei interjects and sarcastically asks if they should, "settle our differences and live together in harmony for the rest of our days?" After she demands to know why they are all truly here, Jon Snow steps forward and explains that what they are facing is not about, "living in harmony," but simply living. He explains that the White Walkers are coming and that they cannot be reasoned with, and that the citizens of King's Landing will become merely more members in the Night King's army.
Cersei mocks Jon by saying that this will be an improvement to many of the impoverished citizens' lives, to which Jon, angered by her flippancy, retorted that he would not have come were the matter not serious, though Cersei again dismissed the King in the North's claim as nothing but a joke. Cersei, based on what Daenerys is implying as a truce, will not pull back her armies, as she does not trust, "the would-be words of a usurper." At this point, Tyrion interjected, noting that no words would convince Cersei, so they would show her physical proof. Sandor Clegane brings out the crate, pushing it over and releasing the undead wight. Cersei and her allies pull back in surprise and horror as the screeching monster hurtles at them; an instant before it can reach Cersei, Clegane jerks it back with a chain around its throat. The angered wight charges at Sandor as Jaime and Gregor Clegane place themselves between Cersei and the wight; Sandor cleaves the wight in half and severs its forearm, showing Cersei that no matter what method is used to stop it, the wight cannot be killed unless it is burned or destroyed with dragonglass. Jon demonstrates both and proclaims that if they do not stop the White Walkers, the same fate that befell the now-dead wight will befall the rest of the world. Daenerys reiterates that she, like Cersei, did not believe Jon until she saw the undead for herself; when a horror-struck Jaime asks how many she saw, Daenerys replies the Night King's army numbers at least 100,000.
Euron then stands and asks Jon if they can swim, and when Jon replies with a "No", Euron announces that he is taking the Iron Fleet back to the Iron Islands, saying that while he has traveled the world and has seen things that Cersei cannot imagine, the sight of the wight is the only thing that terrifies him. After he leaves, Cersei agrees that there will be no kingdoms to rule if the White Walkers advance south. She accepts Daenerys' truce, but says that Jon must extend the truce as well, telling him that he must remain in the North, not take up arms against Lannister forces, and must remain neutral. Jon tells her that he cannot do so, as he has already pledged his allegiance to Daenerys. Cersei then angrily calls off the truce and leaves, telling them that they will deal with whatever is left of the North once the White Walkers are done with them.
After Cersei departs, Jon is scolded by both Daenerys and Tyrion, telling them that he should have just lied for practical purposes. Jon tells them that he cannot make an oath he cannot uphold, saying that although this was the same mindset that got his father killed, "words stop meaning anything," if they are nothing but lies, and that lies won't win them the war. Tyrion still holds steadfast that they are doomed, but he comes up with a solution: he will go talk to Cersei and reason with her. Daenerys is strongly against the idea, fearing Cersei will have him killed on the spot, but Tyrion says that there is no choice, lest they all go home and the struggles they faced leading up to this point would have been for nothing.
Tyrion arrives in Cersei's chambers, with Ser Gregor present. The Queen accuses him of convincing Jon to swear fealty to Daenerys in order to bring about the goal he has worked his entire life trying to achieve: the destruction of the Lannisters. Tyrion says that he is trying to prevent such an occurrence, citing how he advised Daenerys from outright destroying King's Landing from the start. Cersei mentions how he killed Tywin, but Tyrion claims he regrets it in spite of everything their father put him through. Cersei says that Tyrion's act of killing their father let the floodgates open for Tommen and Myrcella's deaths, something Cersei says would not have happened had Tywin been alive. Tyrion claims that he did not mean for that to happen, and taunts Cersei into ordering his execution at the hands of Ser Gregor, saying that if it weren't for him, their parents and Cersei's children would still be alive. Knowing that she won't kill him, Tyrion says that he is more sorry about Tommen and Myrcella than Cersei knows, to which Cersei says that she does not care about anything except the fact that Tyrion's actions cost them their future. She also states that once she saw the wight, she did not care about the world around her, but only cared for the ones she loved the most. As she says this, Tyrion interrupts by saying that Cersei is pregnant.
This meeting proved to be successful, as Cersei returned to the Dragonpit, claiming that she will send her armies north to fight side by side with Daenerys and Jon's forces in an effort to defeat the White Walkers.
At Dragonstone, Jon Snow discusses strategy with Daenerys Targaryen, Davos Seaworth, Tyrion Lannister, and Jorah Mormont. Jon plans for the Unsullied will sail up to White Harbor with Jon and march to Winterfell while the Dothraki will ride hard up the Kingsroad to meet up with the Unsullied at Winterfell. Jorah thinks it would be safer for Daenerys to ride her dragons up north, but Daenerys agrees with Jon that she should stay by her side and will sail with him up north to Winterfell. The council adjourns.
In the Red Keep, Jaime Lannister discusses their strategy with several bannermen. Cersei Lannister walks in with Gregor Clegane and dismisses them all save Jaime, and reveals that she does not intend to move the Lannister armies north, but instead let her enemies slaughter each other in the North while she uses her armies to regain control over Westeros, ignoring Jaime's protests that whoever emerges victorious in the North will turn their attention to the Lannisters afterwards: the White Walkers to simply kill them, or Daenerys and Jon over the fact Cersei betrayed and left them to die. Cersei then informs Jaime that Euron Greyjoy is sailing to Essos to ferry the sellsword Golden Company to Westeros to make up for her losses. Jaime says he made a promise, however, and, unwilling to break it, warns Cersei that he is all she has left. Disgusted at Cersei's willingness to leave all of Westeros to die, Jaime declared his intention to honour his pledge and join the fight against the White Walkers. Furious, Cersei has the Mountain unsheathe his sword, threatening to kill Jaime if he left her, but Jaime calls her bluff and walks away, riding north alone as snow begins to descend on King's Landing, marking the true beginning of winter.
By far the faction that benefited the most from the parley was Cersei's. Having confirmed that the North (and by extension, the Vale) are allied with the Targaryens and will be joining them there, she knows exactly who will actively oppose her when the Great War is concluded. Furthermore, as Daenerys Targaryen is marching her entire army - Unsullied, Dothraki, dragons and the Westerosi knights she captured at the Goldroad - north to battle the Night King, Cersei will be able to conduct operations in the South with less interference once the Golden Company arrives.
However, Cersei's position is still far from secure; all she holds for the moment is King's Landing and the Crownlands. Euron Greyjoy is her ally, but their alliance is based on an informal and uneasy promise of marriage, which will likely be jeopardized due to Cersei's pregnancy by Jaime. Since the supplies she hoped to plunder from the Reach were burned at the Battle of the Goldroad, she has no provisions to feed the capitol of at least a million people now that winter has come, raising the risk of renewed riots. Dorne, although leaderless for the time being, still has its full military strength and remains in a state of open rebellion against the Lannisters, while the Reach and the Riverlands are both unstable with the extinction of the Tyrells and the killing of all the Frey men.
Finally, should the living win the Great War and Cersei has betrayed their truce, Daenerys Targaryen will come south again and exact vengeance. She has proven that she can win almost any battle as long as she has at least one dragon at her disposal; in addition, unlike the Golden Company, who only fight for gold, Daenerys's troops fight for her out of personal loyalty. On the other hand, if the Night King emerges triumphant, the army of the dead will be swelled by the corpses of all those who died fighting the White Walkers in the North, and the Lannisters will lack the means and knowledge to stop them. Cersei also suffered a loss on a personal level: Jaime's defection in the face of her treachery has lost her the one person she has always trusted.
In the books
In the A Song of Ice and Fire novels by George R. R. Martin, the event has yet to occur. Given that the TV show has surpassed the books, it is unclear if the Parley in King's Landing will be convened at all, and under what circumstances it will be if it is. In the past, Great Councils have been convened to discuss the future of the Seven Kingdoms of Westeros when the line of succession for the Iron Throne was being disputed among House Targaryen, the royal family. This great parley seem to differ from the traditional Great Councils, however.