Power of waking

The power of waking (眠りし心を解放する力) 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". It is also one of the ways to safely access the Realm of Darkness, which Riku and King Mickey use in their efforts to find Aqua.

Kingdom Hearts 3D: Dream Drop Distance
Sora manages to acquire the power of waking near the end of his and Riku's Mark of Mastery exam, but loses it when he is corrupted by darkness and nearly becomes a vessel for Xehanort. Riku then uses the power of waking to dive into Sora's heart and defeat the Armored Ventus Nightmare, saving Sora's heart. <!--the purpose of sora and riku's exam is to gain the power of waking, and riku uses it to wake Sora:

Yen Sid: If we are ever to strike down Xehanort, we need the individuals King Mickey spoke of in his letter. We must lead them out of sorrow and slumber, and back to our world. To do so, seven Sleeping Keyholes must be found and unlocked, and a great power retrieved. As you know, every world is walled off from the next, preventing travel between them. In the past, you could bridge these gaps because the walls were broken, or because you could open special Lanes with your Keyblades. But your new goals, the "Sleeping Keyholes," are harder to reach. You'll recall, in your first journey, that you brought many worlds back from the darkness--but some never returned completely. They still sleep, cut off from all outside channels. Not even the Heartless can enter.

Yen Sid: You must understand, this examination is in no way how the Mark of Mastery is usually found. However, in light of what they must do next... it was a necessity. If Sora and Riku complete their test, by finding the seven doors corresponding to the seven pure lights, they will return home with a new power.

Yen Sid: In your Mark of Mastery exam, you were to unlock seven Sleeping Keyholes. By doing so, you would awaken those worlds from their prison of slumber, and also acquire the power to free a heart from its sleep. Riku, you unlocked those Keyholes within Sora's dreams. Therefore, it stands to reason that you now have the power to awaken Sora's heart. Mickey: You want him to dive back into Sora's sleep? But Master, Sora's heart is down in the darkest abyss. If Riku's not careful, he might just get trapped down there with him. No... I'll go instead. --note: "power to free a heart from its sleep" probably "power of waking" in original JP script

Yen Sid: More than anything, I am grateful to have you both back from Xehanort's deception unharmed. And I am grateful to Lea, whose spontaneous actions turned the tides. I am also deeply sorry, for failing to percieve the danger and throwing you headlong into a perilous test. This experience has revealed many hidden truths, and we must gird ourselves for the great clash with darkness that lies before us. I believe we need a new Keyblade Master, one with a new kind of power. Sora and Riku, you both deserve the honor. However, one of you braved the realm of sleep again to unlock the final Keyhole and save a friend. Riku, I name you our new true Keyblade Master.

Ansem the Wise: Yes. A clue, I hope, to finding yourselves or your lost friends in your hour of need. The heart has always been quick to grow. Each exposure to light, to the natural world, to other people, shapes this most malleable part inside of us. Nobodies are not different from us in that manner. Sora was the only one able to return to his human form without destroying his Nobody. That is a statement to the love in his heart for other people, and the bonds that tie them together. Perhaps...he has the power to bring back the hearts and existences of those connected to him-- to recreate people we thought were lost to us forever. Our most precious treasures--even an empty puppet--the trees of the forest, and the petals on the wind--there are hearts around us everywhere we look. And it does not take superhuman powers to see them. Surely we remember as children the way our hearts made everything seem so shiny, and perfect. Sora has a heart like that--uncorrupted, willing to see the good before the bad. When he sees the heart in something, it then becomes real. When a connection seems broken, he may have the power to mend it. He has touched countless hearts, he has accepted them, and he has saved them. And some of those hearts have never left him--whether they fell into darkness or were trapped there--whether they sleep in the darkness of Sora's heart, or were welcomed into its warmth--they can be saved. All Sora needs to do is be himself and follow wherever it is that his heart takes him. It is the best and the only way. The rest is in there.

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Kingdom Hearts III
It was not until a clash with Vanitas in the Land of Departure that Sora is able to regain the power, using it to restore Ventus's heart to his body.

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. The young Xehanort warns Sora about using the power of waking this much, stating that it's meant to be used for "traversing hearts to reach worlds, not for traversing worlds to reach hearts," and that Sora using it as much as he does comes with a high price.

After the final battle with Xehanort, Sora uses the power of waking to revive Kairi, who had been destroyed by Xehanort, seemingly vanishing and becoming lost in the process.