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Legends has it on the net when the sun and the moon meet in the sky. A Haruno girl will unlock a power sealed within her, that unleashes her darker persona.
After than the evil will come out under great stress, fear, or anger. In this form her power increases, and her genjutsu is deadly.
Image on the left, kisames sword sensed her power. He wanted it, unbeknownst to sakura ( she have no memory or clue of this power) kisame was tailing her. He caught and engaged her. She panicked but fought back. Sakura knew her power was limited because she was still storing her seal, and she may not be able to win. On her last reserve kisame tried to kill her, he was a bit upset that she wasn't as powerful as he though.
Then, inner sakura took over and killed him, taking his sword,and using his chakra to fill her byakugou seal. She took his robe fashioned it into a dress and joined the akatsuki on her own. They excepted as she was able to kill kisame, and they could sense her evil.
After that she left, and the good sakura regain consciences,....inner Sakura sealed the sword in her seal, and hid the robe in sakuras bag.
Would you like to hear this in a fan fiction