Svartalfar
While most elves are viewed as beautiful and generally well-meaning, the Svartalfar (Dark Elves, also called Night Elves or Black Elves) are quite the opposite. Millennia ago, they turned to the Demon Lords of the Underworld for greater power, becoming masters of the dark arts. Perhaps due to corruption from this practice, their skin is ashen black, with ridges and horns. Most dwell deep in Nidavellir, far from the sight of Midgard. They are not known for their kindness.
Appearance
Dark elves are the only elf race not typically regarded as beautiful by other races. Their skin color ranges from ash-grey to black, with horn-like protrusions and ridges on their bodies and parts of their faces, such as the brow, nose, and chin. Their hair may be black, red, or stark white, and their terrifying eyes range among bright red, yellow-green, or violet. It is not known how many of these features stem from the demonic corruption that has plagued them over countless generations. They are also the shortest elven race on average, and have the longest ears, perhaps due to their subterranean lifestyle.
Culture
The majority of the svartalfar dwell in immense underground cities inside of great stalactites and stalagmites, sometimes reaching higher than the tallest human castles. These dark cities are lit primarily by glowing crystals, and sometimes runic flames and glowing mushrooms that are the natural flora of Nidavellir. The dark elves are shunned by all surface-dwelling races, including all other elves, and they like it that way. Their greatest enemies are other subterranean races who compete with them for territory, particularly dwarves and goblins. Sometimes they even dispute with dragons over territory and treasure.
Religion
The dark elves primarily worship various gods of death, particularly Hel. Some dark elf houses worship demon lords, often taking a patron demon lord as their primary deity. This practice is usually shunned, however, for most dark elves like to believe they are better than demons - merely "using" them for greater power. They are aware that demons laugh at this notion, believing the truth is quite the opposite. Thanks to the demonic rituals and demon binding deeply ingrained in svartalfar culture, many dark elves are either practitioners of demonic magic or are at least very knowledgable in it. However, most dark elves turn their noses up at outright necromancy - the reanimation of shambling corpses - preferring to summon and bind actual demons to their will.
Summoned Demons
As mentioned, dark elves consider themselves above demons in general. Though they would not dream of directly defying a demon lord, dark elves have played the games of trickery and temptation with demons for years, and they have mastered the art of winning the demons' bargains. As such, summoned demons in dark elf cities are bound to serve their masters or merely trapped in one location to show the power of a noble house, not allowed to wander as they please. Though many - perhaps all - dark elves have some demonic blood in their veins, the practice of copulating with a demon is highly frowned upon, as it usually means a foolish dark elf fell for a demon's seduction. Also, dark elf demon-kin are no stronger than others at resisting the temptations of their demonic blood, meaning they usually simply become another demon.
Military
Dark elf warriors traditionally wear almost colorless armor in black and shades of grey, in order to blend with their environment. The wealthiest prefer armor and weapons forged from void iron, though they cannot craft such armaments as skillfully as the Dvergar, meaning they must often turn to trade with dwarf clans. Dark elves are masters of the sword, spear, and javelin, and have been known to field cavalry riding bound demonic steeds.
