Great Vortex

Ulthuan is spanned by a network of waystones -- towering rune-carved menhirs that channel and redirect the Winds of Magic. Each waystone collects the raw power of Chaos and directs it to an inward neighbour. Thus are the magical energies of the world inexorably drawn, like the waters of a whirlpool, to Ulthuan's heart. There the magic is drained from the world by the Great Vortex; the last -- and most brilliant -- enchantment of Caledor the Great.

The Great Vortex is centred on the Isle of the Dead, the ancient memorial group of the Elves. It is not a true island, but rather an archipelago of waystones whose arrangements are laid in symbols of mystical significance. These menhirs vavy greatly in size -- some are scarce a dozen feet tall and slim as an Elf, whilst others are as tall as mountains and nearly a mile in girth. Without the latent spiritual energy contained within these waystones, the conjuration of the Great Vortex would have been impossible. The Isle of the Dead exists outside time, beyond the reach of the physical world -- its black-robed guardians keep a grim watch to ensure it remains that way. If an intruder were to be allowed upon these haunted shores, he would find the High Elf mages of old, caught like flies in amber, still chanting their ages-long spells to preserve the balance of the world.

Despite the Great Vortex, magic has left its mark on Ulthuan. Strange lights flicker in the skies, beautiful voices dance upon the wind and waterfalls resonate with otherworldy music. Indeed, Uthuan's entire being is now sustained by magic. During a time known as the Sundering, the continent was wracked and shattered, its remnants saved from the hungry sea by desperate enchantments. Only by harnessing the vast power drawn in by the waystones can the Elves ensure that Ulthuan is not swallowed up by the ocean.

Source

 * Warhammer Armies: High Elves (8th) -- p. 9