Cornerstone of Light

The Cornerstone of Light is a sacred relic of Disney Castle. It is a large sphere made of visible and solid-looking light. It is held in the Hall of the Cornerstone, at the base of Disney Castle. It is the defensive mechanism that defends Disney Castle from falling into darkness. It once held in Timeless River while Disney Castle was being built, on Cornerstone Hill.

Description
The Cornerstone of Light is the centerpiece of Disney Castle. Although it does not stop any Heartless from landing on the world, it stops the Heartless from swallowing the world up into the shadow.

Kingdom Hearts II
In Kingdom Hearts II, the Cornerstone first appears. It is failing to stop Maleficent's power from engulfing Disney Castle. To make her conquest of the world complete, Maleficent sends Pete back in time to when Disney Castle was called Timeless River to steal it. Sora travels back with Donald and Goofy to stop Pete. Pete manages to take it and tries to escape on old Pete's boat. Sora stops him and returns the Cornerstone to its rightful place on the pedistle of Cornerstone Hill.