When the Fourth Great Shinobi War broke out at the end of Naruto Shippuden, Madara and Obito Uchiha (and later Kaguya Otsutsuki)were the main foes to be faced. That said, it was another person entirely who proved indispensable in his services to the enemy side, and revived Madara in the first place. That man was Kabuto Yakushi, until only recently Orochimaru's loyal aide. When Orochimaru seemingly died at Sasuke's hands, Kabuto injected himself with Orochimaru's DNA and eventually gained many of the same frightening powers his master possessed, as well as those of many of his servants, like the Sound Ninja 4. Kabuto was instrumental in the war due to his activating the Impure Resurrection, reviving many dead shinobi including Madara to face down the Shinobi Alliance.
Kabuto eventually faced down Sasuke and a revived Itachi, who had managed to free himself from Kabuto's control. During their battle, Kabuto claimed to have surpassed Orochimaru, adding further proof to his words by mastering Sage Mode, something Orochimaru's couldn't withstand. Orochimaru has since been revived, and after coming to terms with the error of his ways, Kabuto also became an occasional ally to the Leaf Village like his former master. In Boruto, he lives a humble life running an orphanage. With both Orochimaru and Kabuto alive and arguably at the peak of their prowess, it's a fair question to ask if Kabuto truly has surpassed Orochimaru in strength. It's unlikely that they will ever trade blows, but some assumptions can be made.
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Kabuto's claim to have surpassed Orochimaru is not complete hyperbole. Orochimaru's body was too weak to handle the Sage Mode transformation, and the fact that Kabuto was able to
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