The Abyss is the realm that Lies, sitting like a pustulent sore between the Supernal and the Fallen
The Abyss is the unknowable space between realities, the realm that has shards in every Sleeper soul, the embodiment of everything that cannot be.
It is the antithesis to the Supernal, used by the Exarchs to poison and manipulate the Fallen World and its inhabitants.
The Abyss is the origin of the Lie, Quiescence, Dissonance, and all Paradox. Anything that challenges The Lie draws its power into the Fallen World, and if a mage is not careful - if they do not enforce their Will with Wisdom - then it will warp and twist their spellcasting.