Dark Elf
From Tolkien Gateway
When Melian set her Girdle around the land of Doriath, Thingol's kinsman Eöl left that land, and dwelt instead in the forest of Nan Elmoth. Living under the dark shadows of that ancient forest, and preferring the night to the day, he came to be widely known as the Dark Elf.
Eöl's epithet was no doubt influenced by the fact that the Sindar to whom he belonged were of the Moriquendi, the Dark Elves who had never seen the light of the Two Trees of Valinor.
