He found Dorothy's Ruby Slippers after they fall on his mountain. Using them, he conquered the Emerald City, turning its inhabitants into stone, with the exception of the Scarecrow, who he captured and took to his mountain. When Dorothy, Tik-Tok, Jack Pumpkinhead, and the Gump arrived at his mountain, the Nome King transformed the Scarecrow into an ornament. He told Dorothy that the emeralds were originally his and that the Scarecrow stole them from him, but she knew he was lying because the emeralds were there before the Scarecrow became king. The Nome King decided to have them all play a guessing game with each of his ornaments to see which one the Scarecrow was, in which if they guessed right, the Scarecrow would be restored, but if they guessed wrong three times, they would be turned into ornaments.
Whenever one was turned into an ornament, the Nome King became more human in appearance. If all of them were to guess wrong and become ornaments, the Nome King would be completely human. All of them were turned into ornaments except Dorothy, who was able to get the Scarecrow back and realized that people from Oz turned into green ornaments. Furious at this, the Nome King blamed Princess Mombi, who previously had Dorothy locked in her tower and was unable to prevent her escape, so he imprisoned her in a cage to punish her later and then turned into a giant form and attempted to eat them shortly after they found the Gump and Jack. He tried to eat the Gump, but only succeeded in eating the sofa part of him, as they were able to save his head. He then tried to eat Jack, but as he was holding him above his mouth, Billina, who was inside Jack's head since their arrival on the mountain, laid an egg, which fell down the Nome King's throat. As eggs are poisonous to Nomes, this killed him, leaving only a pile of rocks and the Ruby Slippers. His mountain was destroyed afterwards.
Nicol Williamson also played Dr. Worley. Interestingly enough, both have a ruby ring on their finger and are smoking a pipe. The Nome King's mountain collapsing, and he himself crumbling into a pile of rocks after being poisoned by an egg, mirrors Dr. Worley's clinic burning down and his subsequent death, much like Nurse Wilson and Mombi, who are both imprisoned at the film's end.