In Old Empyrean the world was whole, but after the cataclysm Grand Cornerstone was shattered into pieces. The breaking of the obelisk corresponded with the disappearance of countless realms as Aether snuck into the spaces they once occupied. The missing realms were locked away within the shards of Grand Cornerstone, along with many core philosophies and powers once integral to Empyrean but now locked forever.
These cornerstone fragments are now scattered across the world, waiting to be discovered. As each cornerstone is reunited into the physical structure of the central obelisk in Nexus, its contents are reintegrated into the fabric of the world. Some cornerstones bestow conceits upon those who discover them, granting the user powers greater than other Eidolons. Other cornerstones are unlocked to reveal paradigms, fundamental and universal truths of Empyrean metaphysics waiting to be rediscovered. Still others contain entire realms, and when they are unlocked these lands are seamlessly reintegrated with Empyrean as if they had never left.
Sometimes an unprotected, undiscovered cornerstone can be corrupted by the Aether and twisted into a colossus. Colossi are literally monstrous mountains—an entire realm given animate form and filled with rage. The only way to defeat one is to remove and unlock the cornerstone at its center, after which the colossus will fall apart as its natural landscape reasserts itself.
Image is “Fieldmist Borderpost” by Michael Bruinsman, copyright Wizards of the Coast.