Zero-Tails

The is a noh masked leech-like spirit sealed within Amaru.

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 ones 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 it's 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, a agent of the Land of the Sky and the man who orchestrated the revival of the Sky ninja forces.

Trivia

 * Though it bears a numerical tail name, it is not a true tailed beast.
 * Since it is referred to as the "Zero" tails, it is vastly inferior to even the lowest of the actual tailed beasts.