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.
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.
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 his friend back.