Zero-Tails (Kai)

The is a Noh masked leech-like spirit sealed within the Land of the Sky. It only appears in Naruto Shippūden 2: Bonds.

Background
The Zero-Tails is an evil spirit created by the dark thoughts and feelings of the downtrodden people. This parasitic creature will seek out and feed upon any negative emotions within one's heart and allows the spirit to create. When anyone near the Zero-Tails is emotionally unstable, it will inhabit their body and take them over. This process is similar to a jinchūriki using the power of its Tailed Beast, but it does not require a human sacrifice, nor does its extraction kill the host.

Abilities
It has the ability to fire tendrils from its body and to also form them into hands. It seems to also be able to regenerate since its head reattached after being ripped off by Naruto in his Naruto Uzumaki's Jinchūriki Forms#Four-Tailed Form|four-tailed form. It also has a very large mouth that appears under its mask. Large molars grow from inside its mouth and it seemed to have tried to bite Sakura Haruno when she attacked it. It appears to have the ability to read the hearts of others, seeing as it saw Naruto's urge to rescue Sasuke Uchiha, and his pain when he was unable to bring him back.

The Dark Chakra produced by the Leech is derived from the darkness within the hearts of the people around it, and it grants the user of this chakra a great source of power, giving access to high-level techniques without even using hand seals while negating any potential drawbacks those techniques may otherwise bring, as well as powering up the entire Ancor Vantian floating fortress.

Plot Overview
The remnants of the Land of the Sky used the Zero-Tails to discretely create enough dark chakra to power their ultimate weapon, "Ancor Vantian". This dark chakra was also personally used by Shinnō, an agent of the Land of the Sky and the man who orchestrated the revival of the Sky ninja forces. He originally had Amaru's village attacked in order to have her fall into despair, in which the Zero-Tails can then take advantage and possess her, in order to gain more dark chakra, but when she overcame her sorrows with Naruto Uzumaki's help, the beast left her body and ended up in Ancor Vantian. When Shinnō was weakened by Sasuke Uchiha, he fused with the Zero-Tails in order to gain more power, but when Sasuke destroyed the tablet that restrained the beast and Naruto dealing the death blow to it, the beast went berserk and destroyed the entire sky fortress, killing all the soldiers aboard.

Trivia

 * Despite the beast referring to itself as Zero-Tails and Shinnō referring to it as a tailed beast, it is not really one of the original tailed beasts due to the fact that it was not born from the Ten-Tails.It's also noteworthy to comment that, during the movie, when the Zero-Tails latched onto Naruto, the Nine-Tailed Demon Fox reacted aggressively to its presence and attempted to expel it.