Oblivion is entropy itself, a constantly churning force of destruction and madness from which there is no return.
Oblivion seethes beneath everything: the Labyrinth, the Tempest, the Shadowlands, and the Skinlands. It constantly calls to wraiths in the form of their Shadows, telling them to give up and join with it. For every wraith that succumbs to Oblivion, it spreads that much farther, and is growing at a constant, but alarming rate, much to the horror of the wraiths who dwell just above it. Nothing can get rid of it; once it has encroached on an area, it remains there. Just ask the dybbuks of Auschwitz within the Dark Kingdom of Wire.
The servants of Oblivion are the dark spirits known and feared as Spectres, wraiths who have become dark and twisted within the very throes of bedlam. Very few wraiths who fall into Oblivion come out again as anything but Spectres. Wraiths who undergo a Harrowing have a very small chance of coming out again, but many become changed by the experience. Oblivion also manages to poke itself through the Tempest and Shadowlands by way of nihils.
Oblivion is believed to be embodied in the form of Grandmother, but there is some debate as to how accurate this assessment is. Some believe that Grandmother is Oblivion's child rather than the force of entropy itself. This doesn't make her any less powerful, but may be crucial in finding an answer that will stop her advance or even end her existence.