Bequeathing

The Keyblade Inheritance Ceremony is a trial in which a certain Keyblade master chooses a child or an adolescent to inherit the Keyblade. If the Keyblade also chooses the host, then the host may summon his or her own Keyblade at will. This was first seen in Kingdom Hearts Birth by Sleep between Terra and Riku, as well as between Aqua and Kairi.

Procedure
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. If the Keyblade also approves of the heir, it will emit a brief glow of light.

The Keyblade is accidentally inherited when one of a strong heart and will makes contact with it. If the Keyblade glows the same way as when passed down intentionally, it can be safely assumed that this person can now summon the Keyblade willingly.

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.

Aqua's Scenario
The Keyblade Inheritance Ceremony is unintentionally performed when Aqua visits Radiant Garden and comes across a young Kairi, who is 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 decided not to do it.

Terra's Scenario
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, 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 the boy.

Trivia

 * Sora is the only one of the three current Keyblade wielders not to receive the ability to use the Keyblade through this method.