the Deep Mists
The point at which the Gloom gives way to the Deep Mists is generally considered to be when the light of Solus can no longer be seen, even at its brightest shine. But the other major sign is that this is the point at which the Smoke of Oblivion becomes thick enough to sicken the living and reanimate the dead.
While in the Gloom it is possible, though dangerous, to live on the surface of an aerolith, but down in the Deep Mists this is no longer possible. The only things that can survive on the surface seem to be the behemoths, but even they don’t spend too much time down here. Only the undead make their homes upon the surface of aeroliths in the Deep Mists, and they do so without fear or limit.
People make their homes inside their aeroliths when you get down this deep, and they live lives that would be almost unrecognizable by the standards of those that live in the light. Some, like the dwarves, seem to be slowly dying off from the constant struggle to survive. Others, like the drow, have come to thrive in the deepest dark places of the world.