The Svartalfar or dark elves live in Svartalfheim, located on the Norse god's plane.
The ancestors of the svartalfar were ordinary elves (Alfar) who were sent on a mission to map the region that would later be called Svartalfheim. For some reason they never returned to Alfheim. After many decades their descendants visited the king of the Alfar. The king was surprised to see that these elves had black skin and green eyes, since the Alfar all had blond hair, fair skin, and blue eyes.
The newcomers called themselves Svartalfar. They also claimed to be a separate race from the Alfar. This angered the king and he ordered his guards to kill the Svartalfar. The guards managed to kill one of the Svartalfar, but not before he killed their king. The others escaped.
This began a long-running feud between the Alfar(or Ljosalfar as they were afterwards known) and the Svartalfar.
Svartalfar mercenaries were hired by Midhir to kill Atticus in Trapped. In Staked they reluctantly form an alliance with Atticus, the Aesir, and the Tuatha De' Danann to fight against Loki and his allies in Ragnarok.
They have the power to turn to and from smoke. Once injured, the Svartalfar cannot turn back to smoke form.