Phenomenal question! In the 20 years we ran FOTN, we only had 2 adventures that took the adventurers to Niflheim.
The first group needed to retrieve an object of great importance so they travelled along one of the roots of Yggdrasil before attaining Niflheim. There was a large drop and Odin's Noose (magic item that reappears in DotN) got them down safe and sound. The fun part was that they met some of their dwellers that had failed their afterlife rolls. During the adventure one player died in Niflheim, failed his roll, and then continued to play his dweller in Niflheim for the remainder of their stay. He even managed to level up while deceased which was really paradoxical! However when the group had to leave, he couldn't go with them due to the "physics" of Niflheim and the spirit world.
The second group saw me as a player, and I chose to play a druid. I'll never forgive the Norn for starting out the adventure in Niflheim and for about half a dozen play sessions I had
no powers due to the absence of any life within that realm. *shaking fist* damn you!

Anyhow long story short there will be a massive write-up and map for Niflheim in Lords of the Ash (Giants & Gods). Until then there are quite a few fun sources you can look for for details: the Prose Edda, the Vafthrudnismal poem, Baldur's Draumar, Helreid Bynhildr among others.
Short and sweet of it:
- It is a land of pure death, the ground is dust that cannot sustain life
- It is a misty realm as the adjacent realm Hvergelmir creates fog that permeates into Niflheim
- It is pitch dark, and light sources (even magical) are seriously hampered by the fog
- The dead wander about aimlessly and get tormented by corpse swallowers- massive skeletal juggernauts whose purpose is to inflict pain and suffering... picture a borg cube made of bones (hundreds of feet tall). they can also turn souls into Draugar.
- Hel's dominion in in a sub-realm of Niflheim called Helheim- a land of steep black stone mountains (Niflheim realm ore comes from here)
- Helheim is protected by her elite guard known as the Draugar
- There are 3 gates that encircle Niflheim: helgrind, nagrind and valgrind
- Bridge over the river Gjoll is guarded by a giantess/troll named Gjallarbru
- Entrance from Midgard (Gnipahellir) is guarded by the hound Garm
- The living can enter and leave, albeit not easily. The dead cannot leave without Hel's express permission (which is beyond rare).
- Adventures here should be rare and absolutely epic and memorable!
Let us know how it goes!

edit: text will show up in LotA, not DotN