Maelost, called the Ridged Land by the lillends, is a small demiplane, about 120 miles across, that curves back on itself spatially. It is composed of black rock that rises and falls, creating steep ridgeds in the midst of black, silt-choked liquid. Some say this water's actually alive. Cold, black rain falls in torrents and fierce winds buffet travelers.
The primary inhabitants are wormlike valgoss and terrifying, bulbous slaiyiths. The valgoss have a symbiotic relationship with tribes of humanoids called the Hanim, who allow them to live inside them in exchange for powers and protection.
Sources: Tales From the Infinite Staircase, A Guide to the Ethereal Plane