The Darkhearts are physical, living manifestations of the darkness within people's hearts. They have existed for thousands of years and serve the great evil himself, Chernabog, as the main units of his grand army of evil.
Pureblood Darkhearts are natural creatures, born from the evil in people's hearts. They are ink-black in color, with yellow beady eyes. They are more common in places that are close to or saturated in shadow magic. Upon their destruction, they simply disappear in puffs of black smoke, without a heart leaving its body.
Emblem Darkhearts, on the other hand, were originally created from machines that reproduce the process of a heart being consumed by the abyss. Afterward, stolen hearts can become Emblems. They have more varied shapes, colors, and are branded with Chernabog's symbol to differentiate them from the Pureblood ones. They are more common on the worlds within the Realm of Light, and usually mimic the shape of creatures or objects within that world.
Darkhearts, Emblem or Pureblood, possess several key characteristics that define them as such. Most Darkhearts are created when the darkness that resides in a person's heart consumes him or her due to letting themselves descend into villainy, thus giving it shape and form; this process will also form White Husks, which are born from the body and soul left behind when the heart is lost. Being born from black aether, they are mindless and act on instinct, their only goal is to find hearts, and consume them to create more Darkhearts. But what they desire above all are the hearts of worlds, and thus they enter worlds in search of the way into the hearts of the worlds. When they consume these colossal hearts, the remains of the world form new worlds, such as Traverse Town and the End of the World.
At the same time, Darkhearts seek out the users of the Keyblades, since a Keyblade holds the power to lock and unlock hearts. In order to stop the users of the Keyblades, the Darkhearts use the Keyblade itself as a homing beacon, but ultimately desire to take the heart that commands it. Because of this, Keyblade wielders are under constant attack.
The strength of a Darkheart is proportional to the depth of the evil from which it was born. The blacker the heart forming a Darkheart, Emblem or Pureblood, the more powerful and more monstrous the resulting being will be. An exception to this is the Horned King, who retained his human appearance for unknown reasons. This is a contrast to White Husks, which determine rank according to the strength of the heart, and whose appearances become more human as they increase in power. Furthermore, as beings of elemental shadow, the Darkhearts are able to take possession of other beings or even inanimate objects, granting them immense power and often reshaping their very forms to suit their purpose.
The Darkhearts, being mindless, usually have no master to direct their actions and instead act on the instinct to gather more hearts. However, there are some higher beings who can command them. Darkhearts will obey those with an affinity for evil magic and a strong will. If many people attempt to control them, the Darkhearts will side with whomever is most powerful. Those with hearts, such as Maleficent, endanger themselves however, as the Darkhearts are attracted to their hearts and if they show weakness, the Darkhearts will not think twice of consuming their hearts.
On the other hand, higher ranking Darkhearts, like the Horned King, can fully command other Darkhearts without endangering themselves. Also, the Dominion XIII is able to command them as well with their strong wills, but are not endangered of being consumed by them for their lack of a fully developed heart.
Darkhearts make use of the "shadow corridors", inter-dimensional pathways that connect the many worlds. These pathways are located in the Dark Realm of Tartarus, and thus are very dangerous to use if one is not accustomed to the abyss.