Kingdom Hearts (キングダムハーツ, Kingudamu Hātsu?) is one of the major entities in the Kingdom Hearts universe. It is the heart of all worlds, and the source of ultimate power and wisdom. It can be considered an aggregate of hearts. There are other entities in the series known as Kingdom Hearts, although they are artificial replicas of the true Kingdom Hearts.
Kingdom Hearts usually manifests as a bright, heart-shaped moon in the sky, with a darker aura surrounding it. Kingdom Hearts is a source of tremendously-powerful light, and the surrounding aura consists of equally-powerful darkness - in essence, it represents the brightest light shining in the deepest darkness.
According to Xehanort, one who opens the door to Kingdom Hearts will be able to destroy the current universe, called "the World", and create a new universe, called the "Next World" , shaped any way that they desire. This person would be "reborn as something far greater than human", and also "[gain] control over all worlds and all people".
Entering Kingdom Hearts
Opening the door to Kingdom Hearts usually requires the χ-blade, a legendary weapon that co-exists with Kingdom Hearts itself. In ancient times, the χ-blade was broken into twenty pieces, seven of light and thirteen of darkness. As such, to reforge the weapon, seven beings of light and thirteen beings of darkness must clash in combat. While it is possible to forge the χ-blade with as few as 1 being of light and 1 being of darkness, doing so will result in an incomplete and unstable version of the weapon.
Once Kingdom Hearts has been opened, it is possible for those without the χ-blade to enter it as well.
Alternatively, Kingdom Hearts can also be opened without the χ-blade by merging oneself with it, at which point a physical door will appear to allow anyone else entry inside. However, it should be noted that this has only happened with a replica, and not with the True Kingdom Hearts.
The interior of Kingdom Hearts does not appear to be static. In both instances where it has been entered, the interior manifests as a world that is important to the person who opened Kingdom Hearts: Master Xehanort's appears as Scala ad Caelum, the training grounds of his youth; whereas Xemnas's appears as an alternate version of The World That Never Was, his base of operations as head of Organization XIII.
There currently exist four forms of Kingdom Hearts, each with varying appearances and methods of creation. 
Kingdom Hearts of worlds' hearts
This form is located in the Realm of Darkness, and is created by the combined hearts of worlds that have been destroyed by Heartless. Once enough worlds have fallen to darkness, fragments of them gather as the End of the World. The Door to Darkness, granting access to the Realm of Darkness, will appear, and Kingdom Hearts will reside just beyond the door. This Kingdom Hearts manifests as a dark silhouette behind the Door, though its light is made evident when the Door is opened.
Kingdom Hearts of people's hearts
This form is located in The World that Never Was, and is created by the stolen hearts of people, released by defeated heartless. It manifests as a yellow heart-shaped moon with a dark blue aura surrounding it. When it is damaged, the moon has a large purple crater in the center, and the surrounding aura changes into a bright purple.
Additionally, a second instance of this form appears in the Keyblade Graveyard, where it formed from the hearts of slain Keyblade wielders during the Keyblade War. This version appears as a pale blue moon, with no noticeable aura.
Kingdom Hearts of the incomplete χ-blade
This form is located in the Keyblade Graveyard, and is formed when a being of pure darkness clashes in combat with an equivalent-strength being of pure light. This clash simultaneously creates an incomplete version of the χ-blade, a legendary weapon that is normally capable of opening the door to enter Kingdom Hearts. This version is once again a pale blue moon with no clear aura.
True Kingdom Hearts
This form is once again located in the Keyblade Graveyard, and the method is similar to that of the incomplete χ-blade. For this, instead of 1 light and 1 darkness clashing, there must be seven lights and thirteen darknesses clashing. To "clash" can be as minimal as a single swing, and the amount of victors, survivors, or casualties on either side does not matter. Once this process is done, the completed χ-blade as well as the True Kingdom Hearts will appear.
This version's colors are the most dynamic. When initially summoned, its color is a fiery gold. When Xehanort draws power from it or corrupts it with darkness, it takes on a dark blue or purple appearance. When it gathers power to recreate the universe, it is a fiery orange.
In the "age of fairy tales", the world was whole and full of light. People believed the light came from Kingdom Hearts, the heart of all worlds, which was protected by its counterpart, the χ-blade. Due to the secret manipulations of the Master of Masters, his students, the five Foretellers tasked with protecting the light, began to mistrust each other after receiving a prophecy of the coming war. They began competing over the world's Lux, believing it would give them the power to push back the oncoming darkness, but in the end, their efforts collapsed into internecine conflict that embroiled the entire world. This came to a head in the Keyblade War, where many Keyblade wielders fought each other and lost their lives. When the conflict ended, the lost hearts gathered to form Kingdom Hearts above the devastated battleground, which became known as the Keyblade Graveyard.
Between Kingdom Hearts χ and Kingdom Hearts Birth by Sleep
After the Keyblade War, the events of the "age of fairy tales" became legend and it was eventually believed that the cause of the war was that many came to desire the power of light contained within Kingdom Hearts, and they clashed together, wielding the Keyblades they created in the image of the χ-blade. Their greed and jealousy introduced darkness into the world, and in the aftermath of this Keyblade War, darkness covered the world, the χ-blade was shattered into twenty pieces, seven of light and thirteen of darkness, and the "true" Kingdom Hearts disappeared into the darkness. However, the world was restored by the light within the hearts of children, but as many individual worlds separated from each other.
As a Keyblade wielder, Xehanort studied the legends surrounding Kingdom Hearts, the χ-blade, and the Keyblade War. After he became a Keyblade Master, he searched for a way to reforge the χ-blade, summon Kingdom Hearts, and restart the Keyblade War in order to purge the world from the darkness that plagued it.
To further his plans, he extracted the heart of his pupil, Ventus, to create Vanitas. He left Ventus with his fellow former student, Eraqus, with the intention of having him and Vanitas clash in the future to create the χ-blade.
Kingdom Hearts Birth by Sleep
Master Xehanort draws Terra, Ventus, and Aqua to the Keyblade Graveyard, the site of the legendary Keyblade War. During the initial clash between Xehanort, the three friends, and Vanitas, Xehanort summons Kingdom Hearts in the sky overhead.
While Ventus and Vanitas clash, Xehanort faces Terra, who he encourages to embrace the darkness. When Ventus and Vanitas fuse, creating the χ-blade, Xehanort uses his power as a Keyblade Master to extract his own heart and implant it in Terra, with the intention of seeing the results of the new Keyblade War in his new body.
However, Ventus and Aqua's combined efforts succeed in defeating Vanitas and destroying the χ-blade, while the new Terra-Xehanort is defeated by Terra's Lingering Will and is knocked unconscious. As Terra-Xehanort is swept away by the explosion of light from the χ-blade, Kingdom Hearts is covered by clouds and disappears from the sky.
Between Kingdom Hearts Birth by Sleep and Kingdom Hearts
An amnesiac Terra-Xehanort is taken in by Ansem the Wise, and he becomes his top apprentice. Together with the other apprentices, he and Master Xehanort's co-conspirator, Braig, conduct experiments into the heart, which lead to the release of the Heartless into the Realm of Light. They overthrow and banish Ansem, with Xehanort taking Ansem's name and, soon after, the hearts of himself and his fellows, creating his Heartless, Ansem, Seeker of Darkness, and the first Organization XIII, with his Nobody Xemnas at its head. While Xemnas leads the Organization in creating a Kingdom Hearts formed from the hearts of people, with the secret plan of using it to transform the other members into vessels for Master Xehanort's heart, his Heartless manipulates Maleficent into creating and opening the door to a Kingdom Hearts formed from the hearts of worlds.
Ten years after the clash at the Keyblade Graveyard, Maleficent and her alliance of villains seek out a way to reach Kingdom Hearts, in the hopes of gaining immeasurable power and knowledge and dominion over all worlds.
To accomplish this, they search for the Seven Princesses of Heart, maidens that possess hearts of pure light who, when brought together, will reveal the Final Keyhole leading to Kingdom Hearts. Additionally, they release hordes of Heartless into many worlds so they will consume the worlds' hearts. They destroy many worlds, and the hearts gather in the Realm of Darkness to create Kingdom Hearts. Although they manage to capture all seven Princesses, each of the villains is defeated by Sora.
Ansem, now possessing Riku's body, creates a Keyblade of heart from the hearts of the Princesses. However, the Keyblade and Final Keyhole are incomplete, due to Kairi's heart residing in Sora's body. Sora appears to drive Ansem off, but after he uses Ansem's Keyblade to free Kairi's heart, Ansem reappears and uses the completed Keyhole to travel to the End of the World, while the others flee Hollow Bastion.
Sora, Donald, and Goofy return to Hollow Bastion, and Sora locks the Final Keyhole, stopping the great influx of darkness that was emerging from it. They then travel to the End of the World, formed from the remains of the worlds taken by the Heartless. Deep within, at the border between the Realm of Light and the Realm of Darkness, they find Ansem.
After a long battle, Ansem reaches the Door to Darkness, beyond which lies the Realm of Darkness and Kingdom Hearts. Ansem forces the door to open, expecting for Kingdom Hearts to fill him with the power of darkness. However, the light of Kingdom Hearts overwhelms him, destroying him. But even with his defeat, the darkness that surrounds Kingdom Hearts threatens to overwhelm the Realm of Light, along with an immeasurable horde of Heartless. To stop the Heartless threat, Sora closes the Door to Darkness with the help of Riku and King Mickey, who become trapped on the other side. The world hearts that were taken by the Heartless are freed, and the worlds are restored.
Kingdom Hearts 358/2 Days
Once Roxas is inducted into Organization XIII, their plan to collect hearts in order to create their own Kingdom Hearts begins in earnest. Zexion explains to Roxas that once Kingdom Hearts has acquired enough hearts, it will have the power to make everyone in the Organization whole again. However, unknown to the rest of the Organization, except for Xemnas and Xigbar, this plan is actually a ruse to ensure cooperation. Xemnas actually plans to use the power of this artificial Kingdom Hearts to turn the Organization into vessels for fragments of Master Xehanort's heart, as part of the plan to recreate the χ-blade.
After Roxas and Xion collect many hearts, Xemnas shows the other Organization members the Kingdom Hearts that has appeared over the Castle That Never Was and how close they are to finishing it. It grows to be five times its original size by the end of the year.
When Roxas defeats Xion, and she begins fading away, she asks him to make sure Xemnas does not achieve his goal of obtaining Kingdom Hearts, and to free the hearts they captured.
Kingdom Hearts II
With Roxas and Xion gone and Sora awakened, Organization XIII begin manipulating Sora into collecting hearts for Kingdom Hearts. They release Heartless into many worlds, forcing Sora to fight and defeat them. Saïx eventually reveals this plan to Sora at the Battle of the 1000 Heartless, in order to throw Sora's heart into turmoil. Although Sora continues to fight the Heartless, he resolves to stop the Organization's plan.
Upon reaching The World That Never Was, Sora and company see Kingdom Hearts floating overhead. It can be seen from anywhere in the Dark City and in the Castle That Never Was.
While they ascend the Castle That Never Was, Ansem the Wise attempts to encode Kingdom Hearts as data. However, the machine is unable to process the many hearts within, and it explodes, damaging Kingdom Hearts and sending Ansem to the Dark Margin. Countless hearts rain down from Kingdom Hearts over the Dark City, spawning Heartless. When Sora's group confronts Xemnas atop the Alter of Naught, Xemnas laments the broken Kingdom Hearts, stating he must start all over again. When Sora, Riku, and Mickey refuse his command to gather more hearts, he attacks Sora, but is defeated.
In order to gain more power, Xemnas merges with his Kingdom Hearts. A new door appears, resembling the Door to Darkness, allowing Sora and his companions to enter Kingdom Hearts and confront Xemnas. Even with the power of Kingdom Hearts, Xemnas is finally defeated. The ultimate fate of his Kingdom Hearts is unclear.
Kingdom Hearts 3D: Dream Drop Distance
The artificial Kingdom Hearts created by Xemnas appears in the sky above Riku's version of The World That Never Was.
Kingdom Hearts III
Xehanort orchestrates the second Keyblade War at the Keyblade Graveyard, pitting the seven Guardians of Light against the thirteen darknesses of the True Organization XIII. As a result, he is is able to recreate the completed χ-blade and he summons Kingdom Hearts with it. Sora, Donald, and Goofy pursue Xehanort into Kingdom Hearts, while the remaining guardians of light hold off Kingdom Hearts from the outside.
After a long battle with Xehanort, he prepares to use Kingdom Hearts to remake the universe. However, before Kingdom Hearts's power can be unleashed, Eraqus convinces Xehanort to surrender. The hearts of Xehanort and Eraqus depart together to the afterlife, while Sora and his friends use the χ-blade to close Kingdom Hearts.
In Quadratum, on the roof of skyscraper, the Master of Masters observes the spherical moon, and compares it to Kingdom Hearts.
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Notes and references