Keyhole

A Keyhole is a 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 will enter it and eventually 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.

Kingdom Hearts Birth by Sleep
When Aqua travels to the Throne Room of the Land of Departure with Ventus's comatose body, after being named the only Keyblade Master successor of Master Eraqus, she finds her late Master's Keyblade. Remembering that Eraqus had instructed her to lock the Keyhole of their home away should evil forces seek to abuse neutral ground between light and darkness, Aqua seals it and uses its power to transform the land so only she knows its layout. Once done, Aqua finds the Throne Room transformed into the Chamber of Waking, which she then leaves Ven's comatose body in. Leaving her new home, Aqua sees the newly created Castle Oblivion, before seeking out Terra.

Between Kingdom Hearts Birth by Sleep and Kingdom Hearts
Years later, Xehanort finds the Hollow Bastion Keyhole in the basement of the castle. As he steps through it, he sees the heart of the world and is fascinated of its power, to the point of obsession. He would later cast off his body and become a Heartless due to this same obsession.

He then spent an entire decade traveling to other worlds and destroying them by sending his legions of Heartless into the Keyholes to devour their hearts. Countless worlds were lost to the darkness without remnants of them gathering to create new ones, such as Traverse Town and End of the World. Until Sora's own travels through the worlds, all Keyholes remained in danger of having the Heartless intrude and devour the hearts within them.

Kingdom Hearts
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 island's 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 the Final 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 using the Keyblade of People's Hearts, 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 then 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.

Kingdom Hearts: Chain of Memories
After defeating Marluxia, Sora leaves the room they were in and a keyhole appears on the doors, which he seals with his Keyblade, causing them to disappear. It is unlikely that this was Castle Oblivion's actual Keyhole though, as scenes from Birth by Sleep suggest that it would be located within the Chamber of Waking.

Kingdom Hearts 358/2 Days
The term "Keyhole" was only mentioned when Roxas and Xion experience a memory of Sora sealing the Keyhole of Agrabah. Coincidentally, all of the devices that Xion absorbs power from during her battle with Roxas were planted near the Keyholes of Wonderland, Halloween Town, and Agrabah. It is unknown if the Organization did this on purpose due to spying on Sora during his first journey or simply because the areas offered more space to fight.

Kingdom Hearts II
Instead of sealing the Keyhole of a world, Sora opens the world's Gate, which reveals a new path to a different world. Curiously, though several never-before-seen worlds appeared in Kingdom Hearts II, not one of them had their Keyholes sealed, mainly because after End of the World was destroyed, all worlds went back to what they used to be, and the newer worlds therefore had no reason to be sealed by a Keyblade.

Sora did use the Keyblade to seal a Keyhole on Pete's door to Timeless River, to unlock the seal on the door blocking himself, Donald, Goofy, and Auron from escaping Cerberus, and to unlock the Underdrome in order to save Megara. He also opens the door to Kingdom Hearts with the help of Riku and King Mickey, to reach the final battle with Xemnas.

Kingdom Hearts coded
Data-Sora needs to find the data Keyhole in order to fix the data world source of Bug Blox corruption. Doing so starts a boss battle with the data version of the main enemy from the recreated world.

Kingdom Hearts 3D: Dream Drop Distance
Yen Sid tasks Sora and Riku to go to Realm of Sleep, where they shall unlock seven keyholes of sleep, to awake their respective worlds from a deep slumber, for them to go back to the Lanes Between. Dream Eaters are beings of a mystical race who resides in this realm, and they will either help sowing good dreams, or wreak havoc spreading nightmares, a severe threat to the sleeping world.

Kingdom Hearts Birth by Sleep

 * Land of Departure: The Keyhole is in the Throne Room, behind the middle throne.

Kingdom Hearts

 * Destiny Islands: The Keyhole takes the form of a wooden door with no knob in the Secret Place. However, the Keyhole itself appears in a flashback. Riku opened the Keyhole, destroying the islands.
 * Wonderland: The Keyhole is appropriately hidden inside the mouth of the Doorknob. It is revealed after defeating the Trickmaster.
 * Deep Jungle: The Keyhole is hidden at the top of the waterfall rock face in the Cavern of Hearts, behind a flock of butterflies. The pathway into the waterfall rock is shown after Stealth Sneak is defeated.
 * Traverse Town: Traverse Town's Keyhole is located in the water fountain in the Second District. It only opens up when someone pulls the rope on the district's bell thrice (three times), causing the water fountain to change face. To seal it, one must vanquish Opposite Armor.
 * Agrabah: The Keyhole is in the final room of the Cave of Wonders, in the Lamp Chamber. After stealing the lamp from Aladdin, Jafar's used his first wish to reveal it. It will not be sealed until Jafar is defeated. (It is later visited by Roxas and Xion during one of their missions to this world.)
 * Atlantica: The Keyhole is hidden in Ariel's Grotto. It can only be accessed using the Crystal Trident or the magic of King Triton's trident. After defeating Ursula, Sora was asked to seal it by King Triton. Unlike other Keyholes, this one does not appear on a soild surface; it floats in the water.
 * Halloween Town: The Keyhole was hidden under Oogie's Manor without his knowledge as he had no idea where the Keyhole was. After it collapsed in the following battle, moonlight reflected on the spot where the Keyhole was. Unlike the rest of the Keyholes, this one was gigantic, forcing Sora to improvise when he sealed it.
 * Neverland: Neverland's Keyhole is located on the clock face of Big Ben. It only reveals itself once the minute hand moves up to the strike of twelve along with the other clockfaces. One must correct the clockface below Wendy to make it match the other ones to reveal the Keyhole.
 * Olympus Coliseum: The Keyhole was hidden under a stone block in the Lobby. It can only be accessed once Sora, Donald and Goofy use the yellow Trinity Mark to remove the stone block. Unlike other Keyholes, Sora doesn't directly send a beam of light into it to seal the Keyhole; he holds up the Keyblade and a blink of light seals it.
 * 100 Acre Wood: The Keyhole is on the clasp that closes the book. It seals itself automatically after completing the story within the book. (It is still sealed in the following game, despite being an optional world)
 * Hollow Bastion: The Keyhole is located deep in the basement of the Hollow Bastion castle, and was found by Xehanort years ago. Sora himself never finds it, and it is only mentioned in the Ansem Reports.
 * Final Keyhole: The Final Keyhole is located in the Great Hall area in Hollow Bastion. It cannot be sealed until the second trip. However, in order to seal it, Sora must vanquish the Behemoth guarding it.

Kingdom Hearts coded

 * Destiny Islands: The data Keyhole is behind the waterfall.
 * Traverse Town: Traverse Town's data Keyhole is in 4 pieces held by Huey, Dewey, Louie, and Data-Riku.
 * Wonderland: The data Keyhole is inside a cage.
 * Olympus Coliseum: The data Keyhole is in the entrance to the coliseum in the Vestibule.
 * Agrabah: The data Keyhole is in the Lamp Chamber behind one of the blox.
 * Hollow Bastion: The data Keyhole is in the Grand Hall.

Kingdom Hearts 3D: Dream Drop Distance

 * La Cité des Cloches: The Keyhole of sleep is inside the catherdal, in one of roof leveled windows.
 * The Grid: The Keyhole is located in the arena Sora fights Rinzler and frees him from CLU's brainwashing. For Riku, it's at The Portal to leave The Grid.
 * Prankster's Paradise: The Keyhole is located on the beach outside Pleasure Island's carnival. For Riku's it's in Monstro's mouth, where Geppetto's boat is.
 * Country of the Musketeers: The Keyhole is located in the courtyard. For Riku, it's backstage of the opera house after defearing Hole Mole.
 * Symphony of Sorcery: The Keyhole is located in the Sorceror's Loft A.K.A Yen Sid's room.
 * Traverse Town: The Keyhole is located in the Fifth District, after Joshua explains Young Xehanort is responisble for splitting the world into two different versions.
 * Destiny Islands: The Keyhole is located beneath the ocean, following the battle with Ursula.

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