Dark Elves
The term Dark Elves is used in different meanings in Norse nythology and fantasy fiction.
In Norse mythology
Norse mythology distinguishes between three kinds of elves: ljósálfar 'light elves', dökkálfar 'dark elves' and svartálfar 'black elves'. It is not certain whether the last two are actually different classes; also, either may be identical to dwarves.
In Tolkien's legendarium
In the legendarium of J. R. R. Tolkien, the term Dark Elves (Quenya Moriquendi) is rarely used, and refers to those Elves who never went to Aman: the Avari, the Nandor and the Sindar. They are only "dark" in not having seen the light of Aman, not "dark" in the sense of evil.
In modern fantasy fiction
In modern fantasy fiction, especially in role-playing games, the term Dark Elves refers to the notion of a subrace of Elves who have fallen to evil. Such Dark Elves occur, among others, in D&D and in the Warhammer world. A similar concept are the Blood Elves of the Earthdawn RPG and the World of Warcraft.
See also Drow language.