Zero-Tails

The is a dark leech-like spirit with a noh mask sealed within Amaru.

The Zero-Tails is a spirit which grew out of the dark thoughts and feelings of the downtrodden.

The process of implanting the Zero-Tails into a human host does not require a Human Sacrifice (at least it wasn't brought up), nor did it's extraction kill Amaru, the girl whose body it was implanted into.

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 remnants of the Sky Country 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 Sky Country 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 significantly inferior to the actual Tailed Beasts.