Cúma

Underworld
The far side of Avaloros is untouched by the light of Phoenix.
Tempest herds the wind onto the living face of Avaloros as Leviathan maintains the form of the edge sea. This dark and desolate land is essentially wasteland without plants or animals on its surface. There are a few openings to the underdark, but only those who can survive the low air pressure can pass onto the surface. One of the few structures that stands in this barren land is an Illithid fortress that houses an elder brain. Refugees from another plane, they seek dominion within the deep underdark labyrinth atop their purple-worm mounts.

    The void beneath the world, known as Cuma, is a barren wasteland. Grey dust sits silently under a dark sky, there is no wind or water. A single fortress exists, hosting an elder illithid colony that seeks dominion over the lower reaches of the world, Martelmar. Two spirit worlds simultaineously exist in the void of Cuma, a peaceful realm for the world's spirits to rest between incarnations and a violent realm to serve as purgatory for the world's spirits who misuse Avaloros' gift of harmony.

Ethereal rest. Un-world
a hill with two glowing mushrooms, that bloom and alternate illumination of the worldin silver and gold. The fallen shrooms of previous blooms are harvested for sustenance and building materials and textiles. The hill is surrounded by a ring of standing stones that exist in the material, and contain the inscriptions of those whose spirits remain in balance. A nearby hill hosts a beautiful city, and others are free to wander ghostly forests, plains, and coasts. Nymphs and elementals come here to rest when not working to bring habitable clime to the above-world.
Illithid fortress
Containing the elder brain, this fortress is the homeland of mindflayers in Avaloros. Elite guardians ride purple worms and defend the fortress from monsters who dwell deep in the core of the land.
Shadow torment
Spirits doomed to purgatory are sent to the underworld and see only it's fell-shade.    

  Travel

Mortal spirits.
Transform into the form of an animal which has some power of locomotion, e.g. bird, fish, snake. This animal-spirit is then capable of traversing to the realm of the dead.
Psychopomp.
Unwitting or otherwise unwilling spirits are assisted in this journey by the psychopomp, a celestial nymph given this specific task. Red horses, ferrymen, swan,
Over-sea.
Crossing over the edge of Pelenen and Feirenen will lead one to the shores of Cuma.
Through-land.
It is said that deep caverns exist that connect all the way to the void of Cuma.