The Cornerstone of Light (光のいしずえ Hikari no Ishizue?) is a sacred relic of Disney Castle that appears in Kingdom Hearts II and Kingdom Hearts II Final Mix. A large sphere made of crystal or glass that sits atop an ornate pedestal and contains swirling light, the Cornerstone protects Disney Castle from Darkness and those who embody it too heavily, represented by the fact that Maleficent was unable to make a physical appearance next to the Cornerstone. It is basically the castle's home security system.
It can be found in the Hall of the Cornerstone, a hidden room under Mickey Mouse's throne and accessible only by pressing a hidden switch under the arm of the throne.
Although it is a powerful artifact and heavily protected, Maleficent takes 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 steal 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. Maleficent's trademark thorns surround the Cornerstone and she appears in a vision 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. The Cornerstone is also the object that allows Sora to open the gates to Port Royal and Atlantica.
During the events of Kingdom Hearts 3D: Dream Drop Distance, Yen Sid surmised that the increasing return of Master Xehanort caused great darkness to spread and gave Maleficent and Pete the darkness necessary to bypass the Cornerstone's barrier and capture Queen Minnie.