Zero-Tails

The is a noh masked leech-like spirit sealed within Amaru. It only appears in Naruto Shippūden 2: Bonds.

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

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.

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 Uzumaki in his four-tailed transformation.

Trivia

 * Despite the beast referring to itself as Zero-Tails and Shinnō referring to it as a tailed beast, it is not considered one of the original tailed beasts due to the fact the Ten-Tailed Beast was split into nine different chakras by the Sage of Six Paths, not ten. However, this movie was made over a year before the revelation of the Ten-Tails, so it is unlikely the film makers were aware of the Ten-Tails. But the Zero-Tails said that it was born from the darkness of others, making it not a true tailed beast, but as strong as one.