Power of waking
The power of waking is 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.
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. It was not until a clash with Vanitas in the Land of Departure that he is able to regain it, using it to restore Ventus's heart to his body.
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 forever in the process.
