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.
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, 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. 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 by completing Kingdom Hearts, the Organization members will become whole again.
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.
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.