The Abyss is a gulf of unreality that divides the Fallen World from the Supernal Realms. It is the primary force that prevents sleepers from awakening.
According to the myths of the Diamond Orders and the Seers of the Throne, the Abyss was created long ago at the end of the First Wizard's War when the Celestial Ladder was destroyed. The effects of this event are called the quiescence. This event would have prevented all awakenings if it were not for the Oracles who built the five watchtowers in the five supernal realms: The Aether, Arcadia, Pandemonium, the Primal Wild, and Stygia.
The Abyss is inhabited by strange, malicious unworldly beings. One example of which is the Prince of 100,000 Leaves. It is said by the Guardians of the Veil that the Abyss grows with every paradox created by a mage.
