Luthien, often called 'Luthien Tinuviel', as Beren called her (Nightingale, daughter of twilight in Sindarin), fairest of the elven maids of Beleriand, lived in the First Age of the Sun before the War of Wrath. Her story and fate is tied inevitably to Beren son of Barahir, Edain (human) lord of Dorthonian, whom she fell in love with when he wandered into Doriath. Luthien Tinuviel was daughter of the great King Thingol of Doriath, greatest of the Teleri elves, who would not give his daughter freely, especially to a mortal man. So, Upon Thingol's discovery of Beren's presence in his land, he sent for him and, having sworn not to harm the man, set before him a quest to recover a Silmaril from Morgoth's iron crown. Upon the successful completion of this quest, Beren would be aloud to marry Luthien, as was their want. So, Beren set out upon his quest while Luthien, imprisoned by Melian the Maian queen of Doriath to stop her from following Beren into hell, devised a means of escape from her prison in order to follow her love. Beren travelled to Nargothrond and there gained the help of King Felagund while gaining strong enemies in the Sons of Feanor (q.v.). Beren and the party left Nargothrond and travelled north disguised as orcs until they came to Wizard's Isle and were imprisoned by Thu, Lord of Wolves. Luthien flees Doriath to help Beren and, with the help of Haun, great hound of the Valar, destroy Wizard's Isle and free Beren (Felagund and his companions had died in captivity at the hands of Thu's wolves). Beren and Luthien wander until they approach Doriath and Beren steals away from Luthien while she sleeps and goes to Angband to fulfill his quest. Before approaching Thangorodrim Luthien and Huan once again find him and, with the help of Luthien's elvish magic, they approach Angband in the guise of a werewolf and bat. They enter Angband and steal a Silmaril from Morgoth's crown while he is enchanted by Luthien. Beren loses the stone, however, when the great wolf Carcharas bites off the hand of Beren that holds the Silmaril. It is regained, however, in Doriath, when Carcharas is killed and Beren in the end fulfills his quest to Thingol.
References
Tolkien, J.R.R. "The Silmarillion"
Ballantine Books, copyright 1977
Tolkien, Christopher (ed.) "The Lays of Beleriand"
Houghton Mifflin Co., copyright 1985
Day, David "Tolkien, the Illustrated Encyclopaedia"
Mitchell Baezley Publishers, copyright 1991
Hypertextual System by FMI Publishing, 1995