The Great Crystal has been for millennia a measure of the balance of power in the world. Prior to The Fall, the High People used it to ensure peace between the rival kingdoms of men, to erect cities from whole cloth, and to end wars with a single word.
The Great Crystal is believed to be a massive lodestone with enormously concentrated magic power. The High People house it in the Praxim of the Ancients at the heart of the God Tree.
The origins of the Great Crystal and its true nature can only be speculated upon. After The Fall, the magic contained within it was unleashed upon the world.