Kingdom Hearts
Kingdom Hearts is one of the major objects in the Kingdom Hearts universe. It is the heart of all worlds, and the source of great power and wisdom.
Kingdom Hearts
In the first game, Xehanort's Heartless as well as the horde of Disney 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. These maidens held hearts that had absolutely no darkness within them. When brought together, these princesses would reveal the final keyhole and the way to Kingdom Hearts itself. Thus, Maleficent attacked many worlds with the Heartless, and captured all seven (one of whom was from Maleficent's very own world, Aurora).
All the while, they were being used by Xehanort, who had been the first to discover the nature of hearts as well as Kingdom Hearts itself. It was also he who sent Kairi to locate the Keyblade master of the realm of light, from the theory that the princesses and the Keyblade wielder were connected.
At Hollow Bastion, Xehanort's old residence, the seven princesses are finally gathered, but only when Sora sacrifices himself to release Kairi's heart, using a Keyblade constructed from the hearts of the six other maidens. However, this inadvertently opened up the way to Kingdom Hearts itself. With Xehanort having gained possession of Riku's body, forcing Riku's heart to wander to Kingdom Hearts, he disappeared behind the darkness that poured out from the keyhole, on the way to Kingdom Hearts.
After Sora is restored from his Heartless self, he once again goes to Hollow Bastion, and seals the Keyhole, holding off the great darkness. Regardless, Xehanort remained a great threat, so Sora and co. ventured to the End of the World, a place created out of the remains of the worlds taken by the Heartless (it is, literally, the "Nobody" of worlds). There, the Heartless thrive, and beyond it lay the door that would lead to darkness, as well as Kingdom Hearts.
After a long battle, Xehanort, wounded, reaches the endless abyss, where the Door to Darkness lay. Xehanort forced it to open, hoping for Kingdom Hearts to fill him with the power of darkness. However, to his horror, Kingdom Hearts is not darkness, but light. The light overwhelms him, destroying him. But even with his defeat, the darkness that surrounds Kingdom Hearts threatens to overwhelm the realm of light. Thus, Sora is forced to close the Door to Darkness, trapping Riku and King Mickey on the other side, in order to stop the Heartless threat. Nonetheless, the sealing also restores the many worlds, along with the barriers between them.
Kingdom Hearts II
Kingdom Hearts is again the goal of Xehanort, but this time, it is his Nobody, Xemnas, and the Organization he leads. Maleficent, revived, also sought out Kingdom Hearts once more.
However, Organization XIII wanted Kingdom Hearts for an entirely different reason: they wanted to use its power in order to become whole, for they led an incomplete existence as Nobodies. To fulfill their goal, Xemnas used Sora (since Roxas was no longer an independent entity), having him destroy Heartless. The hearts that are released by Sora's Keyblade (and presumably all other Keyblades) would then wander to Kingdom Hearts, which floated over the Nobody world as a heart shaped moon, The World That Never Was, assimilating into it. When it is finished, the Organization would be able to become full humans once more.
This plan is nearly realized until Ansem the Wise attempted to seal Kingdom Hearts away with a digital encoder. However, the machine is unable to process something as powerful and unpredictable as Kingdom Hearts, and it explodes, Still, this explosion left a giant hole in Kingdom Hearts, causing countless hearts to rain down over the world, becoming Heartless as soon as they touched the darkness. With Kingdom Hearts, Xemnas needed to start all over again, though the threat of Sora forced him to take action.
In order to gain the power to defeat Sora, Xemnas entered Kingdom Hearts, taking its power. Luckily, a new door, much like the Door to Darkness, appeared, allowing Sora and his companions to enter Kingdom Hearts as well. But Xemnas had already taken Kingdom Hearts' power, and attempted to use it against his opponents. Though he tried his best, he failed. What ultimately happened to Kingdom Hearts after Xemnas' defeat is unknown.
