The Third Kazekage was Sunagakure's most powerful leader ever, though his actual name remains a mystery. What set him apart was his ability to blend magnetic chakra with iron powder to create the Iron Sand technique—basically his own take on the sand manipulation that Shukaku possessed, but amplified through his unique powers. This made him nearly untouchable in combat since metallic weapons couldn't touch him.
Sasori had other plans. He killed the Third Kazekage and converted him into a Human Puppet, his favorite one at that. The effort required to capture and transform someone that strong speaks to how formidable he really was. Sunagakure had no idea what happened to their leader and launched desperate search efforts that likely helped trigger the Third Great Shinobi World War. Once the war kicked off, the village gave up looking and appointed a Fourth Kazekage instead, though search parties would occasionally return to the case over the years.
The truth finally came out during Sasori's fight with Sakura and Chiyo. Sasori brought the Kazekage puppet into battle, and after an exhausting confrontation, Sakura managed to destroy it—and with it, what remained of the Third.
Content compiled by AnimeList.moe from publicly available sources.
