Cornerstone of Light

The Cornerstone of Light (光のいしずえ) is a sacred relic of Disney Castle that appears in Kingdom Hearts II. It is a large crystal sphere filled with swirling lights that sits upon an ornate pedestal. The Cornerstone protects Disney Castle from worlds that are evil, preventing villains such as Maleficent from entering the castle. It can be found in the Hall of the Cornerstone, a hidden room under Mickey Mouse's throne, which is accessed by pressing a hidden switch under the arm of the throne.

Kingdom Hearts II
Although the Cornerstone is a powerful artifact and heavily protected, Maleficent is able to negate its power by taking advantage of a door that appears into Disney Castle-exile Pete's past after he is made miserable by her constant abuse. Using the door, Pete is sent back in time to when the castle had yet to be built, and the Cornerstone sat unprotected on a hill. There, he attempts to destroy the Cornerstone, weakening the defenses of the present Disney Castle and allowing Darkness to penetrate the castle; the Heartless then invade and attempt to take the castle and its inhabitants prisoner. As Maleficent's trademark thorns surround the Cornerstone, she appears as an apparition to taunt Queen Minnie about her imminent victory.

Sora, Donald, and Goofy receive a distress call and arrive just in time to help Queen Minnie raise a defense against the invaders. After a quick trip to Hollow Bastion to seek Merlin's help, they discover the time loophole that has allowed Pete and Maleficent to weaken the Cornerstone's protection. By taking a trip through a new door that appears in the Hall of the Cornerstone that leads to Timeless River, Sora and his party manage to thwart Pete and Maleficent's plans and restore the Cornerstone of the present to full power.

Kingdom Hearts 3D: Dream Drop Distance
Despite the Cornerstone, Maleficent and Pete are later able to invade the castle and take Minnie hostage during Sora and Riku's Mark of Mastery exam.

Notes and references
Pierre Angulaire de Lumière Eckstein des Lichts