Bequeathing

The Keyblade Inheritance Ceremony is a trial in which a Keyblade wielder on the level of a Master chooses a person to inherit the power to wield the Keyblade. If the Keyblade accepts the inheritor selected, that person will become able to summon their own Keyblade whenever attempted. An inheritor can also be accidentally made, if one with the right heart touches the weapon; this is how Kairi became Aqua's inheritor at Radiant Garden. The only intentional ceremony seen is performed on Riku by Terra, setting Riku's fate as a Keyblade bearer.

Procedure

"In your hand, take this Key. So long as you have the makings, then through this simple act of taking, its wielder you shall one day be. And you will find me, friend—no ocean will contain you then. No more borders around, or below, or above, so long as you champion the ones you love."

—Terra passing the power to Riku in Kingdom Hearts Birth by Sleep

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Terra chooses a young Riku as his successor

A Keyblade Inheritance Ceremony may be either intentional or accidental. To pass down the Keyblade intentionally, a Keyblade master must choose a successor. All the master must do is test the strength of the one they have chosen by allowing him or her to hold their Keyblade.

The Keyblade is accidentally inherited when one of a strong heart and will makes contact with it.

Prominent Keyblade wielders that practiced both cases of the Keyblade Inheritance Ceremony are Aqua and Terra. A chosen Keyblade wielder is capable of seeing the door to the heart of a world, as Riku saw in the flashbacks in Kingdom Hearts.

Story

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Kairi unintentionally inherits the Keyblade from Aqua

Some time before the Mark of Mastery exam, when Terra, Ventus, and Aqua were still training together, Ventus felt down about losing to Terra in a mock battle. Terra cheered him up, performing a mock Keyblade Inheritance Ceremony in which he recited the mantra and passed his Wooden Keyblade on to Ventus.

The Keyblade Inheritance Ceremony is unintentionally performed when Aqua visits Radiant Garden and comes across a young Kairi, who was being attacked by a swarm of Floods. Aqua's Keyblade glows when Kairi runs to hide behind Aqua out of fear, implying she gains the ability to summon the Keyblade at this point.

Aqua also had the intention of performing the ritual with Sora when she met him in Destiny Islands, but she noticed that Terra had already made it with Riku. Not wanting them to fight like she and Terra would, she refrained, although Sora and Riku would inevitably clash with each other several times ten years later.

The Keyblade Inheritance Ceremony is intentionally and correctly performed by Terra when he meets a young Riku on the Destiny Islands. Seeing himself through the boy, someone who would do anything to protect his friends, Terra proceeds to select Riku as his successor. Many years later, this showed that Sora was not originally the Keyblade's chosen wielder when he received it. However, due to Riku's descent into darkness in Kingdom Hearts, the Keyblade was led to choose Sora, as Ventus's heart, the heart of a Keyblade bearer, resided within him. Riku would not regain the Keyblade until one year later on his path to return to the light.