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After her fight with sasori and their encounter on the bridge Sakura decided to join Orochimaru, under Tsunade's overs, she told him that she would heal his arms and swear alliance to him, if he let her she Sasuke. He agreed.
1 year later Sakura learned everything she needed to know
Orochimaru: looks like you've become quite the kunochi Sakura, you surpassed and perfected jutsu from the sound 5, Kabuto, and you seem to rival Tsunade chan in medical prowess, who needed them, when I could've had you all along, your body would've did just fine...but what did I expect from the disciple of the Sannin...of Tsunade.....( Orochimaru said that while secretly sending a snake to poison her)
Sakura: looks like you've caught onto my plan. Thank you for training me I have become very strong, I've mastered genjutsu, and I don't require a flute or summons to channel my chakra properly. Beside from Tsunade's training I have become very adapt in my tai jutsu and my strength thanks to fatso, that two headed freak taught me sealing techniques , and my speed and reflexes are greater due to that spider guy....their document ofcourse . Kidommaru was interesting though, his DNA works simular to my yin seal....ku,ku,ku,kuku, Tsunade sama sent me here to spy...I learned these jutsu for my own personal gain....how did I develop it so fast you ask, well thanks to my chakra control and some of that hashirama DNA you had, it was compatible with my seal, so I converted it to chakra and...filled this baby up. And about the curse mark...why would I want that weak snake chakra when I can wield the power of the forest.
Lol, don't mind the text and I messed up on the arm