Kingdom Hearts

Kingdom Hearts (キングダムハーツ) is one of the major entities in the Kingdom Hearts universe. It is the heart of all worlds, and a source of great power and wisdom. It can be considered an aggregate of hearts.

Early history
In the "age of fairy tales", when the World was whole, Kingdom Hearts was believed to be the source of all light. It was protected by its counterpart, the χ-blade. 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, the χ-blade was shattered into pieces, darkness covered the World, and Kingdom Hearts disappeared within the darkness.

Before Kingdom Hearts Birth by Sleep
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 recreate the World as a balance between light and darkness.

To further his plans, he extracted the heart of his pupil, Ventus, to create Vanitas. He left Ventus with his fellow former student, Master 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 unconcious. 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. Assisted by Master Xehanort's co-conspirator, Braig, Xehanort overthrows Ansem, banishes him, and takes his name.

Together with Ansem the Wise's other apprentices, they conduct experiments into the heart, which result in the release of the Heartless into the Realm of Light, the loss of the apprentices' hearts, and the creation of Xehanort's Heartless, Ansem, Seeker of Darkness. Ansem begins manipulating Maleficent into searching for Kingdom Hearts.

Kingdom Hearts
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
Organization XIII worked to collect Hearts by destroying Heartless, in order to create its own Kingdom Hearts. Xemnas shows the other Organization members how big their Kingdom Hearts is and how close they are to finishing it. Large enough, in fact, that the world can be seen outside the windows of the Castle That Never Was's Grey Room as it slowly grows whilst Roxas and Xion gather hearts. The hearts released by Roxas and Xion's Keyblades wander to Kingdom Hearts.

Kingdom Hearts II
Maleficent, now revived, seeks out Kingdom Hearts once more as its growth continues following Roxas's defection.

However, Organization XIII wants Kingdom Hearts for an entirely different reason: they want to use its power in order to become whole, for they lead an incomplete existence as Nobodies. To fulfill their goal, Xemnas uses Sora to destroy Heartless and complete Kingdom Hearts. When it is finished, the Organization would be able to become complete once more.

This plan is nearly realized until Ansem the Wise attempts to digitize Kingdom Hearts and seal it away with a digital encoder. However, the machine is unable to process something as powerful and unpredictable as Kingdom Hearts, and it explodes, sending Ansem to the Dark Margin and restoring Riku's original form. However, the explosion leaves a giant hole in Kingdom Hearts, causing countless hearts to rain down over the world, becoming Heartless as soon as they touch the surface of the Dark City, with other Nobodies, mainly Dusks, attempting to catch the falling hearts. With Kingdom Hearts damaged, Xemnas needs to start all over again, though the threat of Sora forces him to take drastic action.

In order to gain the power to defeat Sora, Xemnas enters Kingdom Hearts, taking its power. Thankfully, a new door, much like the Door to Darkness, appears, allowing Sora and his companions to enter Kingdom Hearts as well. Xemnas had already absorbed Kingdom Hearts' power, and attempts to use it against his opponents. Though he tries his best, and nearly defeats them, the combined efforts of Sora and his friends bring about his downfall. What ultimately happens to Kingdom Hearts after Xemnas' defeat is unknown.

Kingdom Hearts 3D: Dream Drop Distance
After Riku defeats Anti Black Coat Kingdom Hearts appears in the sky above The World That Never Was.

Trivia

 * According to Nomura in Director's Secret Report XIII, the differing colors of Kingdom Hearts imply the nature of its assemblage. The Kingdom Hearts made by Organization XIII, glowing yellow, was assembled from the "hearts of men", meaning people's hearts were compiled into it. In Kingdom Hearts Birth by Sleep, the χ-blade unlocked Kingdom Hearts from "the hearts of worlds", glowing blue, meaning the hearts of entire worlds were assembled for its creation.

Kingdom Hearts (Welt)