Thinking out loud here about an upcoming article for UL#4:
The dreams of underworld gods -- be they indigenous to that environment or exiles cast down from the Land Above -- congest the tunnels, passageways, quadrangles, chambers, and caverns of the Land Below as they rise up like a psychic smog, invisibly congealing into supernatural territories called mezzoglunes.
Mezzoglunes are psychic real estate accessible by mortals only through the act of dreaming. For every level below the surface world, there is a 5% chance that a sleeping N/PC will accidentally "wander" into a nearby 'glune.
It is very rare that such non-places are found uninhabited. Ghosts of dead adventurers, for instance, may take up residence in a mezzoglune and are just as likely to welcome a stray dreamer as they are to rebuke him for showing up uninvited. Demons, shadows and other disembodied phantoms are equally attracted to the ambiance of particular 'glunes, altering the misty dream-environments to suit their unearthly tastes in decor.
There are particular risks associated with dreaming in 'glunes. For instance, an N/PC always runs the risk of encountering a being capable of constraining him, so that his dream-self will be incapable of returning to his physical body. With his mind imprisoned, the N/PC's body could become hijacked by a ghost, demon or phantom.
There's at least one chart I need to generate for these concepts: The table that explains the end-result of an N/PC's sojourn in a 'glune, eg. he gleans useful info from a ghost and answers a question; or he ends up a psychic prisoner and his body is now usurped by a demon -- lots of possibilities for roleplay here!