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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. It is debatable that the Dark Elves referred in the case of Eöl not to the Moriquendi but to the eastern elves who hated the sun.
The question of Eöl's title is addressed in the following margin note by Tolkien:
- "[Concerning the name Eöl] Another name from prim[itive] FG - meaningless then and now. But it was not intended to have any meaning in Q[uenya] or S[indarin]. For Eöl was said to be a 'Dark Elf', a term then applied to any Elves who had not been willing to leave Middle-earth - and were then (before the history and geography had been organized) imagined as wandering about, and often ill-disposed towards the 'Light-Elves'. But it was also sometimes applied to Elves captured by Morgoth and enslaved and then released to do mischief among the Elves. I think this latter idea should be taken up. It would explain much about Eöl and his smithcraft. (I think the name might stay. It isn't really absolutely neccessary that names should be significant.)"
- ― The War of the Jewels: Maeglin