From A Wiki of Ice and Fire
The kingsmoot was a traditional ceremony held by inhabitants of the Iron Islands in which kings were chosen by longship captains. The kingsmoots which chose the High King of the Iron Islands were held on Nagga's Hill on Old Wyk. According to Archmaester Haereg's History of the Ironborn, there has not been a kingsmoot in four thousand years. According to Questions, it has been two thousand years.
In antiquity, each of the Iron Islands was ruled by a rock king and a salt king chosen in a kingsmoot by the island's captains. The kings were usually from the same noble house and were often father and son, but sometimes they were from rival houses.
Eventually, the prophet Galon Whitestaff called all of these petty kings and captains to Nagga's Hill on Old Wyk to choose a high king to rule all of the ironborn. Urras Ironfoot was thus chosen as the first High King of the Iron Islands since the Grey King. Some of the famous driftwood kings chosen at the Old Wyk kingsmoots include Urras Ironfoot, Sylas Flatnose, Harrag Hoare, and the Old Kraken, Loron Greyjoy. When a high king died, his driftwood crown was broken and returned to the sea, and his elected successor would then create a new crown.
Once a candidate put himself forth, he was bound to the kingsmoot's decision. Only once in the history of the Iron Islands has a kingsmoot been declared unlawful. When King Urragon III Greyiron died, the king's family called a kingsmoot while Torgon Greyiron, the deceased king's eldest son, was off at sea raiding. The kingsmoot raised up Urrathon IV Goodbrother, whose first act was to put all the present kin of the old king to death. Torgon the Latecomer returned two years later and claimed the kingsmoot was unlawful since he, the previous king's son, was not present to put forth his candidacy as was his right. Since Urrathon was bloodthirsty and his cruelty made him many enemies, the priests and ironborn denounced him and raised Torgon to be the new king.
Torgon and his son, King Urragon IV Greyiron, both ruled without being chosen by a kingsmoot. The drowned men insisted upon a kingsmoot when Urragon IV died, however. This last kingsmoot turned into a bloodbath. Urron Greyiron gathered his men and killed all the other attendees. Urron Redhand declared himself the King of the Iron Islands and set up the rule of House Greyiron, who wore an iron crown instead of individualized driftwood crowns. His line lasted a thousand years until the arrival of the Andals. Urron Redhand's kingsmoot was the last kingsmoot for anywhere between two thousand and four thousand years.
A Feast for Crows
Aeron Greyjoy declares a kingsmoot as the proper way to raise the next King of the Iron Islands after the death of King Balon. This move is made by Aeron to end a potential civil war between the ironborn and to prevent Euron Greyjoy from taking the Seastone Chair.
Several ironborn put themselves forward as a possible candidate.
- Lord Gylbert Farwynd comes first as a candidate, with his three sons (Gyles, Ygon and Yohn) as his champions, promising the ironborn that if he becomes king he will lead them to a new bountiful land across the Sunset Sea, as Nymeria did with her ten thousand ships. His gifts are whale bone, sealskins and bronze.
- Erik Ironmaker, called the Anvil-breaker, comes next carried on a palanquin by three of his grandsons; Dagon, Thormor and Urek. His gifts are silver, bronze and a few steel daggers and blades. Asha humiliates him by asking him to stand on his own two feet, which he is unable to do.
- Lord Dunstan Drumm comes next. His champions are Andrik the Unsmiling and Drumm's sons Donnel and Denys. Dunstan's speech starts well but he talks too long and soon loses the interest of the crowd. His gifts of bronze do not win them back to him.
- Victarion Greyjoy comes next with Nute the Barber, Ralf the Limper and Red Ralf Stonehouse as his champions. He is well received and his gifts of gold, silver and gems add to his supporters.
- Asha Greyjoy comes next with Qarl the Maid, Tristifer Botley and Ser Harras Harlaw as her champions. She does better than anyone expected. Unlike the rest, her gifts are cobblestones, pinecones and turnips. She shows the ironborn these gifts to demonstrate that this is what they have won so far by fighting the North, losing fathers and sons for these "treasures". She offers them a chance for land, peace and victory. She wins much support, but many more support Victarion and it looks like there is about to be a riot between their followers.
- Euron Greyjoy has one of his men, Cragorn, blow the dragon horn, which silences the fighting. Euron then steps forward and claims the horn they heard can bind dragons to his will, and that he knows where to locate three of the creatures. He promises to use them so the ironborn can conquer all of Westeros. His men then pour forward his gifts, a wealth of treasure. Euron is elected the new Iron King.