A Kingsmoot is an ancient ceremony in Ironborn tradition where a new King is selected to rule the islands following the death of the previous King. All the available captains of longships from the various islands of the region come together to make the choosing. Despite it being well-known Ironborn tradition that allows them to choose a new King, the kingship of the Iron Islands has instead been passed down through various lines, including the Hoare and Greyjoy families, for last few hundred years. As such, it has been a long time since a Kingsmoot had been held. As part of their bid for the crown, each candidate addresses the assembled crowd and offers the reasons why they should be King along with gifts to show the spoils of their successful reaving of the land, in the Ironborn tradition, and to exemplify their strength.
It is traditionally held on the island of Old Wyk at the top of a hill crowned by the oversized bones of a legendary sea creature called Nagga who according to legend was slain many, many hundreds of years ago by the Grey King, an old King of the Iron Islands, and its bones form the open-air hall where Kingsmoots are held.
A Kingsmoot is called at the end of the War of the Five Kings to decide who should rule the Iron Islands following the death of Balon Greyjoy. After Balon's death, his exiled brother Euron Crow's Eye returned the very next day and tried to take the throne for himself, being next in succession in the Greyjoy line. However one of Balon's other brothers, Aeron Greyjoy (a highly respected priest in the Ironborn's religion of the Drowned God), refused to accept him as ruler for various reasons. He used his position of religious authority to invoked the ancient tradition of the Kingsmoot (the first for hundreds of years) as a means to prevent Euron from taking the crown and to have the Ironborn choose a worthy ruler for themselves.
During this Kingsmoot the crowd in attendence had to choose between Victarion Greyjoy, Asha Greyjoy and Euron Greyjoy amongst a few others who put themselves forward. Each candidate put forward their cases and showed the spoils of their reaving, but it is Euron that emerges victorious with his promise of conquering the whole of Westoros using dragons and showing the extravagant spoils of his recent journey through the remains of Valyria - a journey well-known to be frought with danger and peril.