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". 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.
Story
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 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.
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.
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.
As a result of tracking his friends' hearts down, Sora was able to send them back in time to their arrival at the Keyblade Graveyard, erasing their previous failure from existence. Time was kept from repeating, thanks to the arrival of the Lingering Will
After the final battle with Xehanort, Sora uses the power of waking to look for Kairi's heart to bring her back. However, as Young Xehanort had predicted, Sora meddling with time through his use of the power of waking takes a toll. Sora vanished into another world as the price.
