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Light & Dark is an ability from Kingdom Hearts II Final Mix. It is only available as an equipment ability belonging to the Two Become One Keyblade. When Two Become One is equipped as Sora's primary Keyblade, any attempt to enter any Drive Form besides Final Form while fighting will redirect to either Anti Form or Final Form. Although this ability seemingly makes other forms inaccessible you can in fact still enter Limit Form if you attempt to enter it when Sora is on his own.