From A Wiki of Ice and Fire
A kingsmoot is a traditional ceremony held by the inhabitants of the Iron Islands where all longship captains choose their new Iron King by casting votes. The kingsmoot is traditionally held on Nagga's Hill on Old Wyk. According to the book History of the Ironborn, there has not been a kingsmoot in four thousand years. According to Questions, it has been two thousand years.
Some of the famous kings chosen at a kingsmoot include Urras Ironfoot, Sylas Flatnose, Harrag Hoare, and the Old Kraken. The last kingsmoot called was a bloodbath. Urron Greyiron gathered his men and killed all the other attendees and set up the Greyiron rule, which lasted a thousand years until the arrival of the Andals. That was the last kingsmoot for anywhere between two thousand and four thousand years.
Once a candidate puts himself forth, he is bound to the kingsmoot decision. Only once in the history of the Iron Islands has a kingsmoot been declared unlawful. When one king died, the king's brother called a kingsmoot while Torgon the Latecomer, the deceased king's son, was off at sea raiding. The king's brother hoped he would be elected in the son's absence, but the kingsmoot raised up Urragon Goodbrother, whose first act was to put all the present kin of the old king to death.
Torgon 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 Urragon was bloodthirsty and his cruelty made him many enemies, the priests and Ironborn denounced him and raised Torgon to be the new Iron King.
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 the followers of these two claimants until the the dragon horn is sounded.
- 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.