Miraculous Shinobi

While battling the Kyuubi the Yondaime plans to seal the demon into himself and his son when the demon pulls one last trick from his sleeve, an unknown jutsu that backfires and sends the Yondaime, his son and the kyuubi into a world where shinobi don't exist. Alone in this new world he takes comfort in the form of a young teen. Crossposted from Ao3 under same name. Rare pair