Konoha's Demon King: Naruto Uzumaki Kibutsuji

While encountering the Snake Sannin, Orochimaru, within the forest of death, Naruto was injected with an unknown serum that Orochimaru thought would kill Naruto and the Kyuubi. However, unknown to Orochimaru, he ended up augmenting Naruto and giving him new powers that made him someone to be feared as much as Madara Uchiha himself!