Zero-Tails

The is a malevolent spirit, created by the dark thoughts and feelings of the downtrodden people, sealed within the Land of the Sky.

Appearance
The Zero-Tails primarily resembles a dark purple leech with a serpent-like body. On its face, it wears a Noh mask that has the kanji and five red hair-like strands sprouting from it. It also has a mouth that can open very wide that contains large molars.

Abilities
Like Kurama, the Zero-Tails has the ability to sense negative emotions, to which it uses to seek out and possess anyone who is emotionally unstable. Soon afterwards, it begins to feed upon the darkness within one's heart and allows the spirit to create. The dark chakra produced by the Zero-Tails grants its host a great source of power, giving them 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 floating fortress. 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 Zero-Tails is also able to instantly reattach itself whenever it lost a part of its body, even its head.

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. What happened to it afterwards is unknown, but with it still on board of the fortress that Naruto eventually destroyed, the Zero-Tails may have been destroyed.

Trivia

 * The Zero-Tails bears some resemblance to No-Face, a character in Hayao Miyazaki's Spirited Away.
 * 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.

Quotes

 * (To Naruto) "Show me the darkness in your heart."