Light & Darkness

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. When Two Become One is equipped as Sora's primary Keyblade, any attempt to enter any Drive Form besides Final Form will redirect to either Anti Form or Final Form.

This will happen whether you have Final Form or not, which makes it an easy way to unlock Final Form - but rather risky, since you can just as easily enter Anti Form instead.