Zero-Tails

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

It identifies itself as a Tailed Beast and it feeds on the darkness in ones heart to create dark chakra. It was sealed within Amaru so it could discretely obtain dark chakra from the people of Amaru's village. After its task was down, Shinnō used the dark chakra to transform into an "Ultimate Body."