To explain what and where Tharvalok is, it's important to first mention the Elemorion, the world of mortals. 
Elemorion is on the surface, where the grass grows among the trees and where the animals and humans dwell, underneath the skies and above the great below. 
But if you took the entire Elemorion and cut it in half, you'd find that its shape is spherical, like a plane wrapped in the shape of a sphere. 
But what is fascinating is that the sphere in question is, in fact, hollow. 
The crust of the mortal world is about twenty kilometers thick, consisting of the surface and the Underworld, a network of caves and caverns, ravines, and hollows beneath the earth where strange things dwell, where snakes go in winter and spirits roam, and where mankind banished dragons and other ungodly things. 
But beneath the underworld, there is a great void, a hollow almost as big as the world itself, where up becomes down and down becomes up,

And the great hollow is named Tharvalok. 

The atmosphere within the hollow is not filled with air,but with a strange form of aether known as the black aether, the dark reflection of the aether that forms the skies above the mortal world. 
In the middle of the hollow, which is also the center of the world, there is a burning sphere, similar to the sphere that forms the sun, but much smaller, around fifty kilometers in diameter, and the name of the sun below the world is Rhigaldir. 
It is held by four great chains forged by Naldir, the god of fire and craftmanship himself, and each chain is connected to one of the cardinal directions. 
One of the chains connected to the north pole of the mortal world, one to the south, one to the far west, and one below the islands in the far east. 
And on the surface of Tharvalok, there are many creatures and plants quite different from the ones that live in the surface world. 
The first layer of Tharvalok, the lowlands of the realm below are cold and dark, and barely any light from Rhigaldir reaches them. They are often referred to as Drakkura or The Blackwoods. 
They are populated by wild carnivorous beasts covered in scales that walk on four legs and hunt with their massive jaws, lined with rows of teeth. They have ram-like horns, but much larger and proportional to their size; they can grow to the size of a larger adult horse, and their lizard tails are covered in thick black bristles. Their eyes are yellow reptillian eyes, and their skin is gray, like the dark rocks around them. They have two large fangs, and the beasts are named Ayog'ul. 
Their screech resembles the most unnerving scream that a human can produce, and can often be heard in the distance by anyone who roams the Blackwoods. 
There are also many snakes and other reptiles in Drakkura, similar to the ones that could be seen in Elemorion, but very often much larger and much more numerous. In fact, so numerous that the pits full of snakes could be found, despite the fact that the Ayog'ul hunt them and are immune to their venom. 
The realm is covered with black thorn bushes as big as trees that grow among the stone and with some types of black Underworld moss, hence the title, The Blackwoods.