Eöl

In the fiction of J. R. R. Tolkien, Eöl, always called the Dark Elf was an Elf of Beleriand.

He was a Tatyarin Avar who lived in Nan Elmoth, nominally part of Doriath. A great smith and friend of the Dwarves of Nogrod and Belegost, Eöl devised galvorn, a black metal of great strength and malleability, which he fashioned into armour that he wore when he went abroad. He forged two great black swords from a meteorite's metal, but had to give the sword Anglachel to Thingol as tribute for living in Nan Elmoth. The other sword, Anguirel, he kept for himself.

He ensnared Aredhel Ar-Feiniel sister of Turgon when she strolled in his forest, and wed her. Their son was Maeglin. Eöl hated the Ñoldor, and therefore refused Aredhel and Maeglin to seek out their kin. Aredhel and Maeglin later left for Gondolin, stealing Anguirel from him, and Eöl followed them. When denied permission to leave with Maeglin, he tried to kill his son. His poisoned dart actually hit and killed Aredhel, for which deed he was put to death.

Other versions of the legendarium
In the published The Silmarillion Eöl is portrayed as a Sindarin Elf with an unspecified kinship to Elwe Thingol, but from very late writings by Tolkien (specifically Quendi and Eldar, published in The War of the Jewels) it turns out that his final view of Eöl was that Eöl was an Avar who descended from the same clan of the Elves the Ñoldor had come from (the Tatyar), but that Eöl hated his Valinorean cousins. Eöl's love for smithying and friendliness to the Dwarves is consistent with Tolkien's view of the Ñoldor, which are described as Dwarf-friends in the First and Second Ages.