Keyholes

The Heartless invaded Destiny Islands, the home of Sora, Riku and Kairi. Not only did they attack the islands in search of the Keyhole, but also to find the Keyblade Master and one of the Seven Princesses of Heart, who is Kairi. The islands were eventually destroyed when the Heartless attacked Kairi and managed to get into the Keyhole (located in the Secret Cave). Sora ended up in Traverse Town by going through a Corridor of Darkness (which opened during the islands destruction), while Riku ended up in Hollow Bastion. Kairi's body was found by Maleficent. In Traverse Town, Sora learned from Leon and Yuffie that the Heartless took over their world by order of Maleficent. Sora, along with Donald and Goofy, decide it is their duty to travel from world to world and seal the Keyholes using the Keyblade. Maleficent and her gang of villains were in the midst of completing a artificial Keyhole in Hollow Bastion by obtaining the seven hearts of the Princesses of Heart. That Keyhole, once completed, would lead them into Kingdom Hearts, the heart of all kingdoms and source for unmeasurable power and knowledge. However, without Kairi's heart the Keyhole remained incomplete. Her heart was actually dwelling inside Sora's. Sora eventually released Kairi's heart, completing the Keyhole. Ansem, who possessed Riku's body, ventured into the Keyhole in order to get to Kingdom Hearts. Sora sealed the Keyhole and then followed Ansem to the End of the World, a place formed from the remains of worlds taken by the Heartless. Deep in the darkness of that place lay the Door to Darkness, with Kingdom Hearts lying beyond it. There, Sora battled Ansem. Ansem them opened the door so he could obtain the darkness inside Kingdom Hearts, but Sora called out the Light from inside the door and Ansem was destroyed. Sora, Riku and King Mickey sealed away Kingdom Hearts by locking the Door of Darkness on both sides. All of the destroyed worlds were restored, including Destiny Islands. It is presumed all the restored worlds had their Keyholes sealed automatically and hidden, so they could not be destroyed again. Curiously, though several never-before-seen worlds appeared in Kingdom Hearts II, not one of them had their Keyholes sealed. A Keyhole is a magical gateway in each world that leads to the Heart of that world. The Heartless invade worlds and search for their Keyholes (which, not surprisingly, appear as large keyholes). Upon discovering the Keyhole, the Heartless enter it and devour the world's heart. After a world's heart is consumed, the world crumbles, and fades from existence. It is said that, if the Keyblade locks a world's keyhole, it can never be opened again.