A dark elf, son
Maeglin was born and raised in Nan Elmoth,
Beleriand. Maeglin was a skilled smithy and miner taught by his
father and by dwarves.
Eöl gave him his name when he reached maturity. Maeglin means "sharp glance" or "sharp gleam". His mother secretly named him Lómion, which means "child of the twilight".
As Maeglin grew he was fascinated by his mother's stories of Gondolin. Maeglin was chiefly interested to know that he was heir to the throne after Turgon, so one day when Eöl was away visiting the dwarves, Maeglin and Aredhel fled to Gondolin.
Eöl chased after them in rage and was shortly executed in Gondolin for murdering Aredhel with a poisoned javelin in Turgon's throne room (the javelin was actually meant to slay his son, Maeglin).
Maeglin became well known throughout Gondolin as a very skilled warrior, blacksmith, and scholar. Maeglin, a dark elf, also had a dark side of his personality: he desired his first cousin, Idril. It was because of this love that Gondolin fell.
One day when Maeglin was mining outside of Gondolin, Orcs captured him. In turn for his life he agreed to tell Melkor where Gondolin was, to become his vassal once they had taken the city. Maeglin would also get Idril Celebrindal. Then Gondolin was attacked full-force from the outside and Maeglin working on the inside to aid the enemy. While the siege was going on, Maeglin ran after Idril and tried to kidnap her and her son Eärendil along the city walls. Her husband Tuor battled with Maeglin and finally overpowered him and threw the dark elf from the city walls.
"...and his body as it fell smote the rocky slopes of Amon Gwareth thrice ere it pitched into the flames below."Thus the story of Maeglin the elf ended.
-Tolkien: Silmarillion, "Of Tuor and the Fall of Gondolin"
Languages of Middle-earth by Ruth S. Noel