Heartless

The Heartless are beings of darkness that manifest in two forms, "Pureblood" and "Emblem". While most Heartless are, in fact, manifested hearts, they behave entirely devoid of emotion, and thus were named "Heartless".
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Pureblood Heartless are natural Heartless, born from the darkness 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 darkness. Upon their destruction, they simply disappear in puffs of darkness, without a heart leaving its body. While Purebloods have less variety, they are far more numerous in quantity. Emblem Heartless, on the other hand, were originally created from a machine designed to reproduce the process of a heart being consumed by darkness. Once that machine was activated, most stolen hearts become Emblems, provided they contain darkness. Emblem Heartless have more varied shapes, colors, and are branded with the 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. Heartless –Emblem or Pureblood– possess several key characteristics that define them as Heartless. Pureblood Heartless are generated naturally from the darkness within people's hearts. They are typically born when a person's heart is swallowed by darkness. but can also be drawn out by a dark being like Maleficent or a source of dark power like the Keyblade of heart. This process will occasionally also form Nobodies, which are born from the body and soul left behind when the heart is lost. Being born from darkness, they are mindless and act on instinct, their only goal is to find hearts, and consume them to create more Heartless. 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, Heartless seek out the users of the Keyblade, since the Keyblade holds the power to lock and unlock hearts. In order to stop the users of the Keyblades, the Heartless 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 Heartless is proportional to the depth of the darkness from which it was born. The darker the heart forming a Heartless, Emblem or Pureblood, the more powerful and more monstrous the resulting being will be. An exception to this is Xehanort's Heartless, who retained his human appearance for unknown reasons. This is a contrast to Nobodies, 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 darkness, some Heartless are able to take possession of other beings or even inanimate objects, granting them immense dark power and often reshaping their very forms to suit their purpose.
The Heartless, being mindless, usually have no master to direct their actions and instead act on the instinct to consume more hearts. However, there are some higher beings who can command them. Heartless will obey those with an affinity for darkness and a strong will. If many people attempt to control them, the Heartless will side with whomever is most powerful. However, those with hearts, such as Maleficent, endanger themselves, as the Heartless are attracted to their hearts and if they show weakness, the Heartless will not hesitate to consume their hearts.
On the other hand, higher ranking Heartless, like Xehanort's Heartless, can fully command other Heartless without endangering themselves. Also, Organization 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 heart.
Heartless make use of the "Highways of Darkness", inter-dimensional pathways that connect the many worlds. These pathways are located in the Realm of Darkness, and thus are very dangerous to use if one is not accustomed to the darkness.

While the Heartless were not researched with the intent of creating a fighting force, their very nature as manifestations of the destructive force of darkness made them naturally suited for conquering and destroying the various worlds. Their natural ability to travel to various worlds through the use of Corridors of Darkness makes it virtually impossible to fully defend a world from Heartless, and the method in which they are created and exist make it practically impossible to cut them off at the source. Even fighting them once they appear is made difficult due to their immunity to standard weaponry.
While sealing a world's Keyhole will save the world itself, it is still possible for the Heartless to appear on the world and hunt the people themselves. For example, even though the great Door to Darkness at The Radiant Garden was sealed by Sora in Kingdom Hearts, the Heartless were able to remain on that world even after the destruction of the End of the World and the restoration of the worlds, and propagated to form a gigantic siege engine of over a thousand Heartless.
The Heartless's natural danger is only increased due to the many villains who seek to harness the creatures' power for their own goals. Many of these villains, such as Pete and Organization XIII, specifically seek to create abnormally strong Heartless by forcing beings with strong hearts to succumb to darkness.
The primary weapon for destroying the Heartless is the Keyblade, as it is both specially suited for manipulating the bonds between heart, body, and soul which create Heartless, and it is the only object able to thwart the Heartless in their goal of devouring a world's heart. However, in the absence of the Keyblade there are several other options. Magic is effective against the Heartless, as are magical weapons. In either situation, invading Heartless can be made disorganized by defeating the major Heartless which leads them, or the dark heart which attracted them to that world. While this does not by any means purge the world of its Heartless, it gives no small respite from their threat. Emblem Heartless that are destroyed by a Keyblade release the heart within them, sending it to a Kingdom Hearts where it waits for its body and soul to be freed, so that the person it once was can be woken once again.
The Heartless reside wherever darkness is abundant, especially the Realm of Darkness. However, they also reside in places such as the End of the World, a collection of the remnants of worlds that is located near the darkness. Another area that the Heartless live in is The World That Never Was, the world of the Nobodies that rests dangerously close to the darkness upon which the Heartless thrive.
Heartless, as they seek and consume hearts, also desire to return to the greatest heart, the heart of all worlds, Kingdom Hearts, that resides deep within the Realm of Darkness. And because of this, Heartless seek immense darkness, enough to completely consume the Realm of Light. The entryway into this place of great power is the Door to Darkness, the door that separates the realm of light and the realm of darkness. However, with the efforts of Sora and the King, the door is closed, which causes the worlds lost to the darkness to be restored, and preventing a massive army of Heartless from pouring into the realm of light.
The forces of the Heartless were extremely weakened after the door to Kingdom Hearts was sealed off, though there were still many left.  it is unknown what exactly happened to the Heartless and if they still pose a threat. It should be noted though that Yen Sid has mentioned that the only true way to destroy all Heartless is to have all people have light filled in their hearts and no darkness, leaving nothing to create a Heartless or something for them to thrive for.