Niflheim

A realm covered in magical mists and a maze that constantly rearranges itself every time someone enters. The realm was originally created by a Dwarf named Ivaldi as a prison for the enemies of the Aesir. However as time went on and the number of inhabitants of the realm only grew, it became evidently clear that it was no longer a prison but a home for all manner of creatures dark and corrupt. It is said the souls of the evil who have not been claimed by the dark gods are sent here to be reborn as maggots. However, since the realm was built within the Ginnungagap, the endless pit, the abyss of abysses; the darkness was that of corruption and the maggots born of this world would begin to consume one another and twas born the very first demons. Ever since its creation the inhabitants have built upwards in an attempt to escape the endless pit, all in vain. This means the farther down you go in Niflheim, the older and more powerful the demons get.

General Features

Population: Demons and the enemies of the gods
Ruler: Various demon lords
Weather: High humidity and generally unpleasant temperature.
Lightning: Being within the endless pit the realm sits in perpetual darkness. Even with lighting, the mists and fogs reduce visibility to a minimum.
Sounds: The light drumming of water on the swamp, the roars of hungry demons, and most of the time eerie silence.
Terrain - Swamplands: Walking in the slimy water is difficult and progress is slow. 
Terrain - Underground: The smooth yet muddy floor makes progress slow.
Terrain - Air: The air is dense with mephitic vapors and is incredibly poisonous. 

Places of Interest

The Swamps

Stagnant, brackish water, emanating putrid and oily fumes. Its depth varies from a few inches to many feet and every now and then there are patches of solid terrain. The swamp floor is so muddy and dense, covered in thick moss that oozes a viscous substance. Finding a strong grip for walking is difficult. The waters are infested by demonic maggots. They are attracted by heat, since the environment is so cold and humid and a light or heat of a body would instantly attract their attention. Hydras roam the murky waters, silently awaiting their next meal. 

Hvergelmir, The Boiling Sea

Born from the liquid gushing from the horns of a great demonic stag, this body of water surrounds the root of the World Tree that extends down into this realm. The sea boils with poison, at the center of which rises a massive column of deadly vapors. The air is toxic, the heat is intolerable, and the thick pall of steam eliminates all possible points of reference.

River of the Dead

Mortals who lived an evil life will find themselves reborn as maggots crawling out of the mud at the banks of the river. Lowly demons gather here to feast upon the maggots to grow in strength until they are strong enough to leave the river and further into the realm. The river itself flows into the swamps of the realm and many maggots, after being eaten a dozen times over, will instead traverse the river into the swamp for a fighting chance at survival.

Maze of Ivaldi

Much of the realm is designed to be akin to an ever changing maze. One path may never lead to the same place twice and makes it near impossible to back track. Caves and caverns constantly shift opening new passages and closing old ones. Because of this much of the realm remains unknown and unexplored.