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Well, first off, you need to have an 100% complete save of Kingdom Hearts on your memory card. Then, you have to have beaten Sephiroth and have the Ultima Keyblade(This I know for sure). You have to have your Final form at its limit, 7(I'm not sure about the other forms, because I had them at their limit when I obtained Final). Afterwards, you keep using Drive(in a place with strong enemies) until you get Antiform. When you're in it, let yourself die(I suggest you do it near strong enemies). When you come to the continue screen, Sora will be in his Antiform. Choose continue and you'll hear a weird voice say, "I'm not ready to lose!" When you come back at the continue point, Sora will be normal, but in your drive it will say, "Antiform". It uses all of your Drive and it DOES NOT LEVEL UP, no matter how many rumors say it does. If anyone has more info on the requirements please post it in comments