: I expected someone to mention Eol (sp?) Wasn't he called a "Dark" Elf? As in "hadn't heeded the summons to the light of Valinor". It's been a long time since I read it but there's some guy - Elf I'm pretty sure - who was featured in the fall of Gondolin story of the Silmarillion. Was he a rival to Tuor? or Earendil? I remember something about him being "bad-hearted" keeping a lady prisoner in hiis woodland home. Wow I really can only remember bits. But someone must know what I'm talking about.
: Anyway - I don't think this is what the original poster meant by the question and it's certainly true what M. Martinez said about free will in Tolkien's world.
: Can anyone enlighten me?

Eol was orginally a sindarin elf of Doriath, but when the girdle of melian was set about Doriath he became dismayed and left doriath and came to live in the woods of Nan Elmoth, where Thingol and Melian had first met. He shunned sunlight prefering the starlight and darkness that Nan elmoth offered. Eol became friendly with the dwarves of Nogrod and Belegost and fashioned an armour known as galevorn. He also hated the noldor because he blamed them for the return of Morgoth and usurpers of the sindarin lands.
It was Aredhel Ar-Feinel, Turgon and Fingons sister whom he trapped in Nan Elmoth and took her to wife, when she left Gondolin in search of the sons of feanor.
She bore to Eol a son called Maeglin which means sharp glance. But Aredhel tired of her life with Eol and desired to go back to gondolin, Maeglin decided to go with her, so when Eol was away at a feast in one of the dwarve cities they rode back to gondolin.
However Eol rode back early and found them gone only a couple of days and he rode after them. They passed through the land of Celegorm and Curufin (Feanors sons), but when Eol passed through the Himlad Curufin waylaid him asking what his errand was, Eol thought Aredhel and Maeglin were on their way to visit Curufin but Curufin told Eol otherwise, and he realised they must be going back to gondolin.
Aredhel and Maeglin made it back to gondolin and Turgon welcomed Maeglin as his sister son. However Eol also found the hidden way to gondolin and was brought before Turgon. Turgon spoke to Eol kindly asking him to stay in Gondolin with his wife and son, but Eol did not want this and went to hurl a spear at Maeglin but Aredhel threw herself in front of her son and was hit with the spear. The wound was quickly healed but she died soon after because the point was tipped with poison.
Turgon therefore sentenced Eol to death and he was thrown of the side of the carag-dur a cliff in Gondolin. Maeglin stood by and did not stop the execution of his father because he became enamoured of Gondolin and of Turgons daughter Idril Celebrindal, so Maeglin forsook his father as Eol actually says before he dies.
However Maeglin grew in Turgons favour and was soon a mighty lord in Gondolin, but Idril knowing that he loved her did not love Maeglin back and he came to hate her.
When Tuor came to Gondolin Maeglin knew that Idril loved Tuor and he came to hate Tuor. Idril and Tuor married and had a son Earendil.
But Maeglin became a traitor to Gondolin because still at that time Morgoth did not know where the city stood. So when Maeglin went mining outside of the encircling mountains, he was captured by some orcs and taken directly to Morgoth, Morgoth then put his fear into Maeglin and made him tell of Gondolins wherabouts. Morgoth promised to spare Idril so that Maeglin could have her when he attacked the city.
when Morgoths hordes did attack the city, Tuor caught Maeglin in his house attempting to abduct Idril, Maeglin actually attemped to stab Earendil but the baby as he was then had some chainmail on and the blow was turned away.
Tuor when he caught Maeglin hurled him over the hill on which Gondolin stood and so he died.
And that is the story of Eol and Maeglin in a nutshell.