Yonensai is the main villain and final boss from Kenseiden.
During the 16th century, a powerful evil force arises and takes over Japan. Yonensai, the Master Warlock, erects his castle in the province of Edo and sends his demonic minions to plague the country, stealing the legendary Sword of the Dragon King and the five Secret Scrolls, which are protected by his warlock servants.
Hayato, a Samurai warrior and member of the family who protected the scrolls, starts his journey to defeat the warlocks and retrieve the sword and scrolls. He travels through each Japanese province and defeats the warlock minions of Yonensai, recovering the scrolls and the sword, using their knowledge to reach Edo and confront Yonensai on his castle.
On his initial form, Yonensai can't be hit directly, as he keeps floating on the top of screen. He will send waves of fireballs which can be destroyed, and each time a wave is destroyed he loses some health. Once his life is depleted, Yonensai morphs into his true demonic form. His face is the weak point, but the player must be careful as boulders drop constantly in the arena and the boss also attacks by shooting energy waves. Once Hayato defeats Yonensai, his castle burns down, freeing Japan from his influence