Yamiyoryu

Yamiyoryu (闇よ流, Yamiyoryu), more commonly known as the Ten-Tails (十尾, Jūbi), is a tailed beast that is currently sealed inside Amaya Mizuno.

Background
Yamiyoryu first came into being in the waning days of the Sage of the Six Paths's life; to prevent the Eleven-Tails' resurgence. The Sage split up its chakra and used his Creation of All Things (万物創造, Banbutsu Sōzō) ability to create ten separate constructs of living chakra, which would become known as the tailed beasts. Some time after being created, the Sage sat down with all the young tailed beasts and told them that they would always be together even when separated, and that one day they would become one entity again with different names as well as forms than they did then when the time came for them to understand what true power was.