In the 312th issue of the Naruto manga, two new villains were introduced. They were called Hidan and Kakuzu, who were jokingly referred to as the “Zombie Duo”. The reason for this is due to the extreme effort it would take to kill either of them. Hidan was the priest of an evil religion. He had gained immortality and the ability to regenerate from any wound, due to the horrific rituals he performed and murders that he committed. Hidan had his head cut off at one point and survived it! He just had it sewn back on.
Kakuzu was difficult to kill due to the fact that he had five hearts. He could steal the heart of an opponent, which would extend his lifespan. In order to kill Kakuzu, you had to destroy all five of his hearts. This was no easy task, as his hearts gave him the ability to use the elemental powers of their original owner. This made Kakuzu a terrifying foe in combat.
In the same way that Naruto had references to Dragon Ball Z, there were parts of the series that were inspired by other famous manga and anime. The Gentle Fist fighting style is clearly a reference to Fist of the North Star, for example. Kakuzu has several attacks that are references to Mobile Suit Gundam, a series that is credited with popularising the “Giant Mecha” series in Japan. All of Kakuzu’s techniques are named after the different robots from the Gundam series.