Darkness

Darkness is a force parallel to Light, an element for Synthesis items, and is also used in the form of attacks in the Kingdom Hearts universe. Darkness has been known to cause people to lose their hearts to it if they have delved too deep into it. Slowly suppressed and conquered by its influence, these unfortunate souls become Heartless and, if their Heart is strong enough, leave behind a body that becomes a Nobody. Those who follow the path of Darkness will usually gain untold power at the expense of something of their former selves.

Though darkness is mostly used by the series' villains, it is not entirely malevolent; friendly characters are able to use the powers of darkness to some effect, ranging from opening the Corridors of Darkness to projecting dark energy blasts (one of Riku's  abilities in Kingdom Hearts II).

Weapons and Special Uses of Darkness
While darkness is often associated with evil, it can be used for the force of good. If one can accept darkness into themselves without allowing the darkness to consume them, they may weild its powers without succumbing to its influences (as in Riku's situation).

King Mickey's Keyblade is a Keyblade from the Realm of Darkness, though this does not mean that it is actually affiliated with Darkness; in this case, it means that it is necessary to close world doors from the side of the Realm of Darkness.

Darkness is also seen being manipulated into attacks by the Heartless and Nobodies, and to a greater degree Maleficent, the members of Organization XIII, and Riku. Sora can also slightly manipulate Darkness in Kingdom Hearts II through his Drive Form, Anti Form, in which the darkness inside Sora that was created when he became a Heartless in  Kingdom Hearts reawakens. Darkness is also an element used in multiple forms of Magic, the most notable being Gravity and Magnet.

Darkness can also be used for transportation. Experienced users of Darkness are able to open a Corridor of Darkness to travel between locations with a great degree of speed, however these are physical corridors of Darkness and prolonged or repeative use of these lanes of travel with often allow Darkness to manifest deeper within a being leaving them more vunerable to its negative effects in the long run.

There are many items that have used for protection against darkness. Some of these items are seen as simple things such as the Midnight Anklet and Champion Belt which boost the equipped character's resistance to dark-element attacks, or the infamous Black Cloak worn by King Mickey, Riku, and the members of Organization XIII which protect the wearer from the negative effects of using the Corridors of Darkness.

Effects of Darkness
Darkness creates and feeds off of primal emotions such as revenge, hate, and greed, making those it effects experience and exhibit these feelings while constantly making those feelings stronger, eventually leading to the effected becoming a Heartless. Darkness can also change the physical appearance of those it effects, up to such extremes as a normal person becoming a Shadow and Riku taking on the form of Xehanort's Heartless.

Known and Notable Darkness Users

 * Heartless
 * Nobodies
 * Ansem
 * DiZ
 * King Mickey
 * Riku
 * Organization XIII
 * Master Xehanort

Darkness as a Destination
When Nobodies (or replicas) are defeated, they fade back into the darkness. It is unknown what it is like at their final destination, or if they retain any consciousness at all.