Power of waking
The power of waking (眠りし心を解放する力, Nemurishi Kokoro wo Kaihō-suru Chikara?, lit. "Power to free a sleeping heart") is an ability introduced in Kingdom Hearts 3D: Dream Drop Distance and plays a key plot element in Kingdom Hearts III, described by Yen Sid as "the power to free a heart from its sleep".
Kingdom Hearts 3D: Dream Drop Distance
Sora and Riku undergo a Mark of Mastery exam that will not only name them Keyblade Masters, but will also grant them the power of waking by unlocking the seven Sleeping Keyholes, thus awakening the Sleeping Worlds. However, Sora loses this power when he is corrupted by darkness and nearly becomes a vessel for Xehanort. Riku uses the power of waking to dive into Sora's heart and defeat the Armored Ventus Nightmare, saving Sora's heart. Then, within Sora's heart, Riku finds himself on the Destiny Islands, where upon listening to three questions and giving his answers, Riku finally awakens Sora.
Kingdom Hearts III
Sora spends his time traveling through the worlds to rebuild his strength in the hopes of reclaiming the power of waking, but it is not until facing off with Vanitas at The Land of Departure that Sora realizes, thanks to Ventus's help, that the power was sleeping inside of him. He then proceeds to use it to free Ventus's heart within his own, awakening him from his eleven year-long sleep.
After restoring himself in The Final World, Sora uses the power of waking to save his friends from the Lich, using it to travel from world to world and track each of his friends' hearts down. Young Xehanort warns Sora about using the power of waking incorrectly, stating that it is meant to be used for "traversing hearts to reach worlds, not for traversing worlds to reach hearts," and that Sora using it in such a way comes with a high price that he has already paid through said misuse.
As a result of tracking his friends' hearts down, Sora is able to send them back in time to their arrival at the Keyblade Graveyard, erasing their previous failure from existence. Thanks to the arrival of the Lingering Will, history is kept from repeating.
After the final battle with Xehanort, Sora uses the power of waking to look for Kairi's heart to bring her back. Upon arriving in the Final World and explaining his situation to Chirithy, it explains that the power of waking is supposed to be used to wake and restore sleeping hearts, not to restore them after fading from existence. Because Sora did that six times already, he changed history which is a forbidden act and if he saves Kairi, he'll lose his powers and vanish from the world. Sora was still willing to do whatever it takes to get Kairi and with advice from Chirity, does just that. However, as Young Xehanort and Chirity had predicted, Sora meddling with time through his misuse of the power of waking takes a toll, causing Sora to vanish into another world as the price.
When Sora used the power of waking to save his friends from Terra-Xehanort's fatal assault at the Keyblade Graveyard, the impact of forcing his friends' hearts back the way they were rewrote reality. This created a singularity, a tear in the fabric of time. The rewrite caused the first version of the battle in which they perished to have never happened. This allowed Sora to relive the battle a second time, where events took a more favorable turn for them.
Kingdom Hearts Melody of Memory
On the search for Sora after he disappeared, the Fairy Godmother takes Riku and Kairi to the Final World, where they meet the Nameless Star. The Nameless Star opens a Portal to the world of Quadratum, and Riku uses the power of waking to travel through it.
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