Svartalfheim or Svartheim is a dark, inhospitable and cold land, the atmosphere itself is grey and gloomy. The Duergar are the inhabitants of this realm. Most of them live far below under the mountains, underground caves and rocky grounds. They claim ownership over this realm. Above live the dark elves, dark Alfar so called Dokkalfar or Svartalfar. Their origins are not from this realm, as such, they are considered immigrants.
There was a great deluge that broke many lands apart. This was the realm where the Duergar made their home and excavated their realms below.
This mountainous, dark and windy realm has woodlands, but the Duergar care little for them. Instead, they made amazing and great halls under the earth, carved on rock. The Svartalfar invaded their homeland, but these two races made a truce deal and both live in this realm to this day, in separate territories of course.
Some times people get confused and call this realm Nidavellir. That is a very common mistake, because the territories of the Duergar are so vast, that it looks like a different realm. Nidavellir is in fact the name given to the territories of the Duergar, but the realm is Svartalfheim. Most of Svartalfheim is Nidavellir, the rest is just territories controled by the dark elves.
As I have written, this is a very dark world and there is little variation between summer and winter, and there isn’t spring or autumn at all. It rains as much as it snows during winter. That kind of weather lessens with the coming of summer, which is more windy at that time. Days are slow, slower than any other realm of the Nine Worlds. However, the sun is barely seen in Svartalfheim, even during Summer. The inhabitants of this realm have a nocturnal existence, such as the fauna. Although, the Duergar venture out of their tunnels during the daylight only, it is safer. But their lives are spent underground in their cavernous cities.
Svartalfheim unlike the other realms, is very easy to enter. The Duergar care little about the business of wanderers and travelers, and it is likewise with the Svartalfar, however, the latter, are more difficult to persuade (but still, easier than the elves of Ljosalfheim). Once you are in Svartalfheim, few are those who will bother you, perhaps it is because they might think in isn’t worth to bother with adventurers, if they entered Svartalfheim, they must know what they are doing (or not) it is a very dangerous place to wander about, there are few shelters and it gets dark too fast, so people are on their own.