Ishido Kazunari was the main antagonist in the Shōgun novel and television miniseries. He was the primary rival of Yoshi Toranaga.
By the time John Blackthrone arrived in Japan Ishido and Toranga were two of the most powerful daimyo in Japan at the time, and both were on the five member council of regents.
Ishido was curious about Blackthorne but Toranga had him thrown in prison as a pirate until he was able to ensure that Blackthorne would be safe from Ishido's grasp.
Ishido had managed to convince the other three council members to support him against Toranaga and was planning to have them all gather at once so they could force Toranaga to commit seppuku. Toranaga resigned from the council, which meant that the council could take no action since they did not have the required five members. Unbeknownst to Toranaga Ishido managed to have someone join the council, bringing it up to full strength. He planned to bring Toranaga before the council to force his rival to commit suicide, but Toranaga was able to escape with the assistance of Blackthorne.
Toranaga and his allies wanted to attack Ishido, but the man had taken many of their families hostage at his castle. Blackthorne and Toranaga's translator Mariko went to Ishido's castle to secure their release. Ninjas hired by Ishido and Kasigi Yabu attacked the castle, killing Mariko and seriously injuring Blackthorne. Even though Mariko had died she was successful in that she was able to force the release of the hostages.
Soon afterwards Toranaga and Ishido's forces met in battle. At the Battle of Sekigahara Ishido is captured alive. In keeping with a prophecy that stated that Ishido would die an old man with his feet firmly planted in the Earth Toranaga orders him buried up to his neck in the ground. He invited locals to come up with a bamboo sword to saw at the most famous neck in Japan. After three days Ishido finally died, feeling very old. Not long after the Emperor named Toranaga the Shōgun of Japan.
Ishido was based on the Japanese daimyo Ishida Mitsunari, who was a rival of Tokugawa Ieyasu (the man the character of Toranaga was based on). In battle Ishida was defeated by Tokugawa's collation and put to death.