Naruto hears Iruka says 'That's what a demon would do' and unconsciously releases Kyubi's chakra, that reacted with 'HIRAISHIN' seal on the forbidden scroll. He was transported to DIMENSIONAL GAP. He starts dying, when Kurama forces his chakra through him to keep him alive and somehow gets him out of Dimensional gap and land in Underworld. Naruto's condition was bad, but he was in luck as he was found by Rias gremory and her queen Akeno himejima. Rias out of kindness uses her mutated Rook to reincarnate him and save his life. Let's see Adventure of Uzumaki Naruto as Rias' Rook.