Kingdom Hearts

Kingdom Hearts is one of the major worlds in the Kingdom Hearts universe, known as the heart of all worlds and has a source of great power and wisdom.

Kingdom Hearts: Birth by Sleep
In Kingdom Hearts Birth by Sleep, set ten years prior of the first game, Kingdom Hearts appears similar to the one in Kingdom Hearts II, but with blue in the sky above the Keyblade Graveyard. The graveyard is what remains of a conflict called the Keyblade War, which was many years before Birth by Sleep. Keyblade Masters affiliated with Light and Darkness, fighting to create the legendary χ-blade, a Keyblade capable of unlocking Kingdom Hearts. These Keyblade Masters failed and destroyed themselves; their remaining weapons littering the world.

Some time after, a Keyblade Master named Master Xehanort plannned to create the χ-blade. In his reports, he explains his views of the equilibrium of Light and Darkness and how they countered those from Master Eraqus's. Opposite to Eraqus's beliefs of light being absolute, Xehanort saw that light couldn't exist without darkness. He noticed that light was prevailing and that darkness was excluded too much and created a goal to destroy the current system of light and recreate a new system of darkness, and be able to reorganize the worlds.

In the end, he succeeded in creating the χ-blade by manipulating his former apprentice, Ventus, and his dark counterpart, Vanitas. With the creation of the χ-blade, Xehanort stopped his battle with Terra, released his heart by striking his chest with his own Keyblade, and used the power of Kingdom Hearts to transfer his heart into Terra's body. After he obtained access to Terra's body, a part of Terra's emotions went into the discarded armor, forming the Lingering Will. Xehanort lost in a battle and fainted, unable to proceed his plans with Kingdom Hearts because of his memories being sealed.

Kingdom Hearts
In the first game, Xehanort's Heartless, Ansem, Seeker of Darkness, and the horde of villains led by Maleficent sought out Kingdom Hearts. Both wanted to draw out the darkness that surrounded it in order to gain immeasurable power and knowledge.

To accomplish this, Maleficent searched for the Seven Princesses of Heart, maidens with hearts that had no darkness within them. When brought together, these princesses would reveal the final keyhole and the way to Kingdom Hearts itself. Maleficent attacked many worlds with the Heartless and managed to capture all seven. During all of this, she was being used by Xehanort's Heartless.

At Hollow Bastion, what remained of Radiant Garden, the seven princesses were gathered, but the path to Kingdom Hearts did not open until Sora sacrificed himself to release Kairi's heart.

Sora traveled to Hollow Bastion a second time and sealed the Keyhole. Xehanort's Heartless was still a threat, however, so Sora and company went to the End of the World, where, in the eternal abyss, lay the unlocked door to Kingdom Hearts.

After battling Xehanort's Heartless, they reached the endless abyss to where the Door to Darkness lay, with the Realm of Darkness and Kingdom Hearts waiting behind it. The door was opened, in hopes for supreme darkness by Xehanort's Heartless, but instead was revealed to be light. The light destroyed him, but the darkness surrounding Kingdom Hearts threatened to overwhelm the Realm of Light. To stop the threat, Sora closed the Door to Darkness, trapping Riku and Mickey Mouse on the other side. The destroyed worlds were restored, along with the barriers between them, preventing any Heartless from invading the Realm of Light.

Kingdom Hearts: 358/2 Days
Throughout the game, Organization XIII tires to collect hearts by destroying Emblem Heartless, to create their own Kingdom Hearts within The World That Never Was. Even though their goal is to become complete beings, Xemnas, Xehanort's Nobody, intends to become one with Kingdom Hearts itself.

Kingdom Hearts II
By the time Sora learns that hearts released by a Keyblade, the Kingdom Hearts above the Castle That Never Was is nearly complete. But other than Xemnas, a revived Maleficent also seeks out Kingdom Hearts once more and refuses to let the Organization get to it first.

Due to Ansem the Wise's attempt to seal Kingdom Hearts away, the explosion that sends him to Dark Meridian and restored Riku's original form left a hole in Kingdom Hearts, causing hearts to rain down over the world and becoming Heartless as soon as they touch the darkness. Xemnas attempted to have the Keyblade wielders help him again, though their refusal forces him to take drastic action to carry out his plan of becoming one with Kingdom Hearts.

A new door, much like the Door to Darkness, appears and allows Sora and his companions to enter Kingdom Hearts. But Xemnas had already absorbed Kingdom Hearts' power and attempts to use it against his opponents after transforming Kingdom Hearts' dimension into a replica of the World That Never Was. What happens to Kingdom Hearts after Xemnas's defeat is unknown, but can be thought that it no longer exists.

Sonic the Hedgehog 4
Kingdom Hearts returns in Sonic the Hedgehog 4, where it has been mechanized by doctor Eggman to make a new version of the Death Egg, which can be seen in Act 3 and the boss act of Sylvania Castle Zone and boss act of White Park Zone. Kingdom Hearts appears to be smaller than the Little Planet. The new Death Egg is a hollow sphere built with another giant spherical structure above and around Kingdom Hearts at the center similar to a Dyson sphere, the hypothetical Dyson sphere is designed to absorb energy from a star and the planetary core consists of the innermost layer(s) of a planet; this, presumably, is also the method which Eggman uses to power his latest space station. Sonic and Tails defeat Dr. Eggman and Metal Sonic and the Death Egg mk.II's power drains out leaving Kingdom Hearts still trapped inside it.