The Great Barrier is a boundary of magic between the Old World and New World, located in the Valley of the Lost. A people called the Baka Tau Mana were forced out of this land when the Barrier was constructed by the Wizards of the Great War under the leadership of Alric Rahl.
3000 years later, Richard destroyed the Great Barrier, allowing the Baka Tau Mana to retake their homeland, and also allowing the Imperial Order to freely begin conquest of the New World.
The Barrier was basically a maze constructed of magic. One who had the gift could be surrounded by it and killed after going through it. The Towers of Perdition are located here, and one, accessible to those with Additive Magic, could reach it and find white Sorcerer's sand. The other was accessible only to people with the gift of Subtractive Magic, and contained black sorcerer's sand.