Ten-Tails

According to Madara Uchiha, the Ten-Tailed Demon is the ultimate fusion of the other nine tailed beasts. It was originally one being, but the Sage of the Six Paths sealed it into his own body, becoming the first Jinchūriki, and saving the world. If the Sage were to die, the beast's chakra would leak throughout the world and cause massive damage. So, upon his deathbed, he dispersed the Ten-Tailed beast into the other nine beasts throughout the world, and sealed its actual body into the moon through the usage of the Chibaku Tensei. Madara's current goal is to capture all nine tailed beasts and merge them back into the ten-tailed beast and become its host.