The Sundering was the drastic division of Flesh and Shadow in the wake of the death of Father Wolf.
In ancient times, the world of matter was not sharply divided from the world of spirit. Instead, there were many hazy transitional areas called Border Marches, where the two could freely mingle. The Border Marches were the native terrain of the Pangaeans, including Urfarah, whose were not wholly of one world or the other.
When the future Forsaken killed Urfarah, his death howl shattered the Border Marches and killed everything inside. In their place rose the Gauntlet, a durable barrier that would forever segregate the Shadow from the material world.