Darkness

Darkness is a force parallel to Light in the Kingdom Hearts universe and is also a form of attack from the Kingdom Hearts series. Darkness is associated with evil due to its nature of deception. People may lose their hearts to darkness if they have delved too deep into it. Driven insane by its corruption, they become a Heartless. Those that follow the path of darkness (villains such as Maleficent and Xehanort's Heartless) gain enormous power, but usually lose much of their physical selves to it.

However, when one with a particularly strong will becomes a Heartless, the shell that they leave behind begins to act with a will of its own, with only minor physical differences from its original person. This being is called a Nobody. Nobodies do not truly exist without a heart, as are shuned by both Light and Darkness, although they are created froma process of Darkness.

Mickey's Keyblade is from the Realm of Darkness, though he follows the path of light.

The Dark element is also used as a form of attack by the Heartless, Nobodies, Riku, and some other enemies. Sora does not use the darkness as a means of attack, unless he is in Anti Form. Because of it's dangerous corrupting effects on the heart, many garments were designed to defend against them, such as the Black Cloak or aMidnight Anklet.

Darkness can also be used for transportation. Users of Darkness are able to teleport (like Maleficent), or open up a Corridor of Darkness to travel between worlds faster than any Gummi Ship (The members of Organization XIII are constantly using Corridors of Darkness to travel distances). The downside of these corridors is that they corrupt the user's heart the longer he/she travels them, which is why frequent users of the corridors who aren't Nobodies or Heartless would wear a Black Cloak to escape the negative effects.

Darkness can also be used for more then just a means of attack and transportation, as it can also alter the appearance of the user. Ansem and Riku are two users of darkness that can change appearance by utilizing its properties. Riku was transformed into Xehanort's Heartless by the power of darkness, while Ansem The Wise used it to become DiZ. It should also be noted that if one stays in the Realm of Darkness for too long, his skin darkens and his eyes turn amber. This is evident with Xehanort's Heartless and his nobody Xemnas, and with Ansem the Wise to a certain extent - Ansem's eyes have turned amber (while in the picture shown in the Door to Darkness, Tron's computer system, Ansem's eyes were originally brown).

Perhaps the most significant process of one losing their heart to Darkness in the Kingdom Hearts universe is that of Organization XIII. It is currently unknown as to particularly when the first six founding members' (Xehanort, Braig, Dilan, Even, Aeleus, and Ienzo) hearts were lost to darkness, however it is most likely because of their research into Darkness and the Heart. Another unknown piece of information is (excluding Xehanort) what happened to the mysterious Heartless of the Organization members. Sometime after becoming Nobodies from the loss of their hearts to Darkness, the six formed Organization XIII, and began recruiting members. Their mission and purpose: to regain their hearts from Kingdom Hearts.