The Subterrane is an extra-dimensional realm in the cosmology of those of Fels. Like Exa, it is accessible to those who wield odylicism by travelling inside the Argence. It is described as endless realm of tunnels, caverns and dungeons, all-pervaded by an eerie yellow light. Whether it is a realm in its own right, or simply a region within Exa is a matter for debate.
The Subterrane is used by those with power and the ability as an inescapable prison for those who have irked them in some way.