The Lower Depths, also called Abaddon by some of the highest archmages, are a sphere of reality that is distanced from the Fallen World in the same manner the Fallen World is distanced from the Realms Supernal. Mages use the term to describe any realm that is neither Supernal nor Phenomenal, but is instead defined by a lack of something.
The light of magic is barely existing within its barren wastes and the ten Arcana are somewhat diminished when it comes to influence the Depths. It's inhabitants, called collectively akarthartoi, are envious of those in the higher realms and feed upon Mana and souls in order to survive when unlucky fortune drives them higher.
As most mages tend to ignore the depths for their own safety, not much is known about them, but several realms are thought to be a part of it and some believe that the Underworld has some connections to it. Also, the Abyss seems to hold no sway over its inhabitants.