Resonance

is episode 70 of the Naruto: Shippūden anime.

Synopsis
The chakra of the Nine-tails rages out of control within Sora, preventing Naruto and the others from subduing him. Asuma and Kazuma resume their old battle. Naruto is repeatedly knocked down in his attempts to restrain Sora, and the chakra seeps into him and transforms him as well. Yamato is unable to suppress the Nine-tails' chakra in both Jinchūriki, being prevented by an unknown outside force. Sora's fourth tail emerges, causing the Nine-tails's Cloak to envelop his body. He temporarily gains his own consciousness when he witnesses Naruto stab himself in the leg and successfully prevent the original Nine-tails' Cloak from emerging.