The Greyjoy kingsmoot ritual from the novels that was greatly reduced in Game of Thrones is the kingsmoot itself.
A kingsmoot is an Ironborn assembly in which leading captains choose a ruler rather than automatically accepting a hereditary successor. In A Feast for Crows, Aeron Greyjoy calls the meeting on Old Wyk after Balon Greyjoy’s death. Asha, Victarion, and Euron Greyjoy compete before a large gathering of captains, with speeches, gifts, competing visions, and religious ceremony.
The television series includes a kingsmoot in season 6, but compresses the event into a short sequence. Yara Greyjoy, Victarion, and Euron make their cases before the Ironborn, after which Euron wins and Yara and Theon flee. Many of the novel’s political details, candidates, speeches, and ritual atmosphere are omitted or simplified.
The kingsmoot is not the same as the Red Wedding or a trial by combat. It is a specifically Ironborn method of selecting a king, rooted in the islands’ older traditions and closely connected with the Drowned God.