Hanzou (半蔵) also known as Hanzou of the Salamander (山椒魚の半蔵, Sanshouuo no Hanzou) was a legendary ninja who was the former leader of Amegakure. He was very security-conscious, being guarded twenty-four hours a day and having anyone who wanted to see him thoroughly searched. He became famous around the time of the Second Great Ninja War. Little is known of his abilities aside from his riding on the head of a giant salamander (presumably a summon), but he was so powerful that he with his summon fought off all three Sannin. After their battle, he was so impressed with their strength that he decided to leave them if they agreed to accept the title of Konoha's Legendary Three Ninja (伝説の三忍, Densetsu no Sannin); This makes him responsible for the naming of the trio's title. Being capable of defeating all the three Sannin head-on, Hanzou is easily a kage-level ninja. His power was such that Jiraiya expressed intense shock upon hearing that Hanzou had been defeated by Pain.

Hanzou's insecurities later got the better of him when a group of ninja, lead by Yahiko, Nagato, and Konan began a quest to end war through unorthodox means. He viewed these ninja as a threat to his power and sought to get rid of them. He then teamed up with the ANBU Root's leader Danzō to kill the group's leader Yahiko in a treacherous manner. He was later single-handedly killed by Pain (under unknown circumstances). The hatred that Nagato had for him for causing Yahiko's death was so severe that he conducted a systematic genocide on anybody even remotely associated with Hanzou (with the exception of Danzou, who somehow escaped the carnage).

(Source: Narutopedia)