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 declared a kingsmoot as the proper way to raise the next King of the Iron Islands after the death of King Balon. This move was 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 came first as a candidate, with his three sons (Gyles, Ygon & Yohn) as his champions, promising the Ironborn that if he became king he would lead them to a new bountiful land across the sunset sea, as Nymeria did with her ten thousand ships. His gifts were whale bone, sealskins and bronze.
- Erik Ironmaker, called the Anvil-breaker, came next carried on a palanquin by three of his grandsons; Dagon, Thormor & Urek. His gifts were silver, bronze and a few steel daggers and blades. Asha humiliated him by asking him to stand on his own two feet. If he could, she claimed she would follow him. Euron laughed at this. Erik tried but could not manage.
- Lord Dunstan Drumm came next. His champions were Andrik the Unsmiling and Drumm's own two sons Donnel and Denys. Drumm's speech started out well but he talked too long and soon lost the interest of the crowd. His gifts of bronze did not win them back to him.
- Victarion Greyjoy came next with Nute the Barber, Ralf the Limper and Red Ralf Stonehouse as his champions. He was well received and his gifts of gold, silver and gems added to his supporters.
- Asha Greyjoy came next, however, with Qarl the Maid, Tristifer Botley and Ser Harras Harlaw as her champions. She did better than anyone expected. Unlike the rest, her gifts were cobblestones, pinecones and turnips. She showed the Ironborn these gifts to demonstrate that this is what they've won so far by fighting the North, losing fathers and sons for these "treasures". She offered them a chance for land, peace and victory. She won much support, but many more supported Victarion and it looked like there was about to be a riot between the followers of these two claimants until the the dragon horn was sounded.
- Euron Greyjoy had one of his men, Cragorn, blow the dragon horn. It silenced the fighting. Euron then stepped forward and claimed the Horn they heard could bind dragons to his will, and that he knew where to locate three of the creatures. He promised to use them so the Ironborn could conquer all of Westeros. His men then poured forward his gifts a wealth of treasure. Euron was elected the new Iron King.