The Abyss is the gulf between the Supernal World and the Mundane World, created at the fall of Atlantis. Most mages view the Abyss as a sort of semi-aware nothingness, a void that nonetheless seems to have an (albeit alien) agenda, and contains, despite its emptiness, a multitude of strange and incomprehensible threats. Merely to study the lore of the Abyss courts madness.
These threats range from actual entities to dangerous energies, from horrifying destructive power to insidious ideas that rend the mind. The impositions of the Abyss may be gross or subtle, instantaneous or ongoing, vicious or seductive. They all lead to destruction, however.
Whether the Abyss is a byproduct of the battle between the mages of Atlantis and the Exarchs, or whether the Exarchs deliberately created it to wall the Supernal World off from the Mundane, the Abyss seems to serve the purpose of the Exarchs. It has drastically slowed the flow of Mana from the Supernal to the Mundane, made Awakening far more rare and difficult, and acts as both the medium and the power behind the Quiescence and Paradox.