It is Jakuho Raikoben, "Hornet Sting of Immortal Thunder"
Her bankai is called Jakuho Raikoben. It takes on the shape of a giant missile . The missile is so powerful that she needs something sturdy to attach to to reduce the massive recoil it creates from the force of the explosion. Soi Fon abhors using her Bankai because of its sheer size and power, which doesn't quite fit well with the whole motif of the Assassination Squad.