"Ukon's Ability" (右近の能力, Ukon no Nōryoku, Viz: Ukon's Prowess) is chapter 204 of the original Naruto manga.
Sakon and Ukon are split apart because of Kiba and Akamaru's attack. They believe they have won, only to find that the brothers easily recover from the separation. They try to attack again, but are blocked. When Sakon and Ukon counter-attack Akamaru puts himself in danger to protect Kiba. While Akamaru manages to blind Sakon at the last minute, Ukon is able to join with Kiba's body. He tells Kiba that he will kill Kiba from the inside, destroying his cells and leaving unfazed once Kiba is dead. Needing to help Akamaru, Kiba stabs himself, taking advantage of the fact that Ukon, merged with him, will suffer the same injuries.