Harry Potter and the Ninja Library

Before her being completely sealed by Team 7 Kaguya used all of her Strength to send Black zetsu into a new world. Hagaromo noticing this wanted to warn the ninja but, after giving Naruto and Sasuke the majority of his powers lacked the strength to return to the mortal world. So in a last ditch effort he used the last of his power to find a suitable soul to reincarnate into the world where Black zetsu was sent.