Lúthien was born in Doriath, only daughter of Elu Thingol, King of Doriath, and Melian, a Maïa,. She was considered the most beautiful of all the Children of Ilúvatar.
She spent the first years of her life in the forests of Doriath, singing and dancing on tunes composed for her by Daeron the bard, the inventor of the Cirth, who deeply loved her.
One summer evening as she was dancing in Neldoreth, she was seen by Beren son of Barahir, Lord of the First House of the Edain, who trying to escape from Morgoth's armies had gone through the Ring of Melian into Doriath. As he did not know her or dare approach her, he named her Tinúviel, the Nightingale, the Twilight-Maiden. Spellbound, he became dumb and could not speak.
In the following spring he heard her sing and this song broke the spell and chased winter away. He called: 'Tinúviel'. She saw him and loved him and they spent spring and summer together.
Daeron, jealous, betrayed them, denouncing them to Thingol who, scorning Men, ordered Lúthien to tell him where Beren was. She only gave in after Thingol promised not to kill him, but she took him herself before the throne as if he was a honoured guest.
Regretting the promise he had made Lúthien, Thingol, to get rid of Beren, promised to give him his daughter's hand if he brought one of the Silmarils from Morgoth's crown. Beren went away and from that day, Lúthien sang not again in the Kingdom of Doriath.
When Beren was captured by Sauron, Lúthien learning what had happened asked Daeron for help to save him. Daeron denounced her to Thingol who, wishing to protect her, locked her up in a house built in the branches of Hírilorn, the greatest of all the trees in the Forest of Neldoreth. Lúthien escaped: With her hair she wove a dark robe and a rope having both the power to put anyone touched them into a deep sleep.
She did not get much far: She met Celegorm and Curufin, sons of Fëanor, who at that time ruled the City of Nargothrond, and was taken prisoner. Celegorm wished to force Thingol to give him her hand. She escaped thanks to the help of Huan, the great hound chief of the wolfhounds of Celegorm he had been given in Valinor by Oromë.
Huan took her to Tol-in-Gaurhoth where Beren was held. He defeated Sauron and Lúthien forced him to give her the mastery of the fortress, which she threw down, freeing Beren.
After these events, Celegorm and Curufin were expelled from Nargothrond. On their way they met Beren and Lúthien and attacked them. But Huan forsooked the service of Celegorm and they were beaten. Beren took the weapons, armor and horse of Curufin as well as Angrist, a knife made by Telchar of Nogrod. As they were leaving, Curufin took Celegorm's bow and shot an arrow, aiming at Lúthien. The first arrow was caught by Huan and the second hit Beren in the breast. Lúthien healed him.
Beren, to protect her, committed her to the care of Huan and departed alone to Angband, but caught up by Lúthien and Huan who, as they had passed by Tol-in-Gaurhoth, had taken with them the wolf-hame of Draugluin and the bat-fell of Thunringwethil. And clad in these, Beren and Lúthien arrived to Angband.
They were stopped by Carcharoth who knew for a long time about the death of Draugluin. Lúthien ordered him to sleep and he fell.
Before the throne of Morgoth, her true identity was revealed. And Morgoth desired her! And he was beguiled by his own malice. Lúthien danced and sang for him and her song had such loveliness and power that all listened perforce. And with her enchanted cloak, she put Morgoth and all those present to sleep.
She awoke Beren who took one of the Silmarils, but when he tried to take another, his knife, Angrist, broke, starting to awake Morgoth. They fled, but Carcharoth had awaken and, attacking Beren, he bit off the hand which held the Silmaril, swallowing both. The pain caused by the jewel put Carcharoth to flight.
But the fangs of Carcharoth were poisoned and the hosts of Morgoth were awakened. Then, bidden by Huan, Thorondor, the King of the Eagles, and his vassals rescued them and bore them to the borders of Doriath.
For days, Lúthien tended Beren and, when hope was almost spent, he woke again.
But it seemed to him unfit that one so royal as Lúthien should live always in the woods, without home or honour. Therefore Beren decided to take her back to her father, who seeing the ordeals they had been through, gave Beren Lúthien's hand.
Carcharoth, because of the Silmaril, went through the Ring of Melian. A battue was organised. Carcharoth felled Beren and bit him at his breast before killing and being killed by Huan. Beren was taken to Lúthien who bid him await her beyond the Western Sea.
Beren died. And Lúthien followed him. And they met in the halls of Mandos. There, Lúthien knelt before Mandos and sang to him "the song most fair that ever in words was woven, and the song most sorrowful that ever the world shall hear. Unchanged, imperishable, it is sung still in Valinor beyond the hearing of the world, and listening the Valar are grieved. For Lúthien wove two themes of words, of the sorrow of the Eldar and the grief of Men, of the Two Kindreds that were made by Ilúvatar to dwell in Arda, the Kingdom of Earth amid the innumerable stars. And as she knelt before him her tears fell upon his feet like rain upon the stones; and Mandos was moved to pity, who never before was, nor has been since."
Ilúvatar then gave Lúthien the choice to live in Valinor until the end of the world without Beren, or to return to Middle-Earth with Beren, but as a mortal. This doom she chose, deciding to be separated from the doom of Elves, she was the only one of the Eldalië to die (apart from Arwen).
Beren and Lúthien dwelt in Ossiriand, on the isle of Tol Galen in the midst of Adurant. Later that region was called Dor Firn-i-Guinar, the Land of the Dead that Live. They had a son, Dior Eluchíl.
No mortal spoke ever with them.
After the murder of Thingol by Nogrod Dwarves and the sack of Doriath, Beren recovered the Nauglamír in which had been set the Silmaril, which he gave Lúthien. Dior left them to rule Doriath. One night of autumn, lord of the Green-Elves of Ossiriand came to him and gave him the Nauglamír. He then knew that Beren and Lúthien had left this world.
"But the wise have said that the Silmaril hastened their end; for the flame of the beauty of Lúthien as she wore it was too bright for mortal lands."
Sil, Chapter XXII - Of the Ruin of Doriath
"None saw Beren or Lúthien leave the world, or marked where at last their bodies lay."
Sil, Chapter XX - Of the Fifth Battle: Nirnaeth Arnoediad