The First and Last Opponent

is episode 266 of the Naruto: Shippūden anime.

Summary
Suigetsu and Jūgo escape from their imprisonment within the Land of Iron due to weakened security, caused by the mobilisation of forces for the war. While searching for his sword, Suigetsu educates Jūgo on the Seven Ninja Swordsmen of the Mist, including previous generation of members and their swords. Meanwhile, the Third Division battles the reincarnated swordsmen, and Kakashi formulates a plan that employs Santa Yamanaka, Ensui Nara, and Maki from his division. Launching the plan of action, Kakashi nearly hits Zabuza, but his attack is intercepted by Haku, who uses himself as a shield. Zabuza cuts through Haku to get to Kakashi, but he is able to narrowly avoid the full force of Zabuza's attack and suffers only a minor wound on his abdomen. After Haku is trapped and sealed, Kakashi attacks once again and he and Zabuza strike simultaneously, but this time Kakashi manages to evade the great blade and pierce Zabuza. Zabuza and Haku are then captured and restrained. With Zabuza's and Haku's defeat, Kakashi commandeers the Kubikiribōchō and declares that he is going to unleash Hell on the enemy forces. Elsewhere, the Second Division continues to fight the White Zetsu Army clones, and Kitsuchi is informed that thousands of the enemy forces had passed them underground--where the current enemy had erupted from. On another battlefield, Gaara discovers one of the enemy reincarnated shinobi.

Trivia

 * In this episode even though Kakashi and Santa had switched bodies, when Santa sensed Zabuza and released the technique, he did so from his own body instead of Kakashi's.