Svartalfheim is a dual land, divided above and below by the two races that live there - the Duergar, who claim primary ownership of the world, but who choose instead to live underground in mountain caverns; and the Dark Alfar, who are immigrants and live partially above the earth and partially under it.
Some people call these immigrants the Dokkalfar or the Svartalfar.
When the worlds broke apart in the great deluge, the Duergar found refuge on a piece of Ymir's body, supposedly his lower spine, and excavated themselves a world. Although trees did grow on the surface of their dark, windy, spine-mountainous world, the Duergar hardly cared. Instead, they dug out amazing halls under the earth, carved with stunning artwork and architecture. When the Dokkalfar moved into the upper surface of the world, they and the Duergar struck a truce deal, with separate territories. Although the Dokkalfar do live in underground dwellings as well, theirs are generally surface caves and mounds, while the Duergar halls extend a mile or more under the surface. They call the Duergar-inhabited part of their world Nidavellir, as opposed to not-Nidavellir, which is what they call anything controlled by the Dokkalfar.
Time and Seasons:
Svartalfheim is said to have the longest year of any of the worlds, discounting Helheim where all time and seasons are artificially maintained by Hela. A Svartalfheim year is equal to several Midgard years. There is little variation between summer and winter, and nothing in the way of spring and fall. The only variations seem to be a greater propensity to rainfall (or snowfall in the higher altitudes) during winter, and more high winds during summer. Days, like years, are two to three times the length of days in other realms; the ratio depends on the realm in question. Even in summer, days are shorter than nights - maybe a little less than half the time - and in winter, the Sun barely shows Her face in Svartalfheim. The Dokkalfar have adapted wonderfully to an almost entirely nocturnal existence, and indeed find the extensive daylight of other worlds oppressive.
The Duergar venture out of their tunnels only during daylight, and most of their lives are spent underground in a world of cavern-cities, which they can make just as bright as they choose, so the outside time and season hardly matters to any of them. The are aware of the seasons, though, in a rather idealized way; when they picture them in carvings and art, they show the lush beauty of the seasons in other worlds such as Vanaheim, Asgard, or Jotunheim. Indeed, the Duergar live almost as if they are pretending they are somewhere else; only a few goatherds venture out during the summer, and a few others on pilgrimages to shrines.
Svartalfheim is more difficult to get into. The dwarves operate by their own rules and have a great many more visitors and travelers going in and out of their realm than the elves do. Once you are inside the elf lands, moving about is actually easier than in Ljossalfheim, as the Svartalfar don't usually care where you go once you've earned their permission to enter in the first place. Svartalfheim is just as magically potent as Ljossalfheim, but it's also a colder, scarier and creepier place. The dark-elves are often malicious and untrustworthy, and few spirit-workers who wander into their realm want to hang around longer than necessary.
Supported by RavenKaldera and Elizabeth Vongvisith ( Spirit-Worker )