Darkness

Darkness is a force parallel to Light, and is also used in the form of attacks in the Kingdom Hearts universe. Darkness has been known to cause the loss of people's hearts if they have delved too deep into it. Slowly suppressed and conquered by darkness's 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. The Realm of Darkness is also one of the three known realms in the Kingdom Hearts universe, along with the Realm Between, and the Realm of Light.

Though darkness is mostly used by the series's villains and is believed to be pure evil, 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). Additionally, Mickey Mouse, a heroic character, uses the Kingdom Key D, which originates in the Realm of Darkness. Nonetheless, different characters have different views on darkness; Aqua regards it as nothing but "hate and rage," while Mickey insists that light and darkness are interdependent and cannot exist without one another.

Nearly all beings have darkness in their hearts, but the levels of darkness and light vary from person to person. For instance, Sora has less darkness in his heart than Pete or Maleficent. There are a few exceptions to this rule, however, as all seven of the Princesses of Heart naturally have hearts completely devoid of darkness. As a result of Master Xehanort forcibly removing the darkness from his heart, Ventus's heart is pure light as well.