Twilight

Twilight represents the realm in between Light and Darkness. It borrows powers from both forces, making it possibly the most powerful of all three. It is relatively unknown as even King Mickey was not aware of it until realizing the path Riku took. This is the force that all Nobodies(including Organization XII, though they have seperate "elements" based on different spells which are the basis of they're powers) draw they're energies from.

DiZ refers to this path as "the twilit road to nightfall" but Riku calls it "the road to Dawn". Xemnas refers to it as "Nothingness". Since the nobodies, who are said to be twilight beings, possess neither light nor darkness it can be assumed that there are two different interperatations of the term twilight: the first being Riku's twilight which indicates a fusion of light and dark, and the second being a broader definition emcompassing all that does not fall under light or dark. Worlds that are tied to both Realm of Light and the Realm of Darkness are known as twilight worlds (The World that Never was and, not surprisingly, Twilight Town are known twilight worlds).

While it is never fully established, it can be assumed that Riku's keyblade "Way to the Dawn" represents Twilight, as he finally realizes his place after Chain of Memories and is finally able to wield it, not too mention he was initially the person too wield the Keyblade of Light(and of course, the name).