The Silmarillion

These were the choices that he gave to Lúthien. Because of her labours and her sorrow, she should be released from Mandos, and go to Valimar, there to dwell until the world's end among the Valar, forgetting all griefs that her life had known. Thither Beren could not come. For it was not permitted to the Valar to withhold Death from him, which is the gift of Ilúvatar to Men. But the other choice was this: that she might return to Middle-earth, and take with her Beren, there to dwell again, but without certitude of life or joy. Then she would become mortal, land subject to a second death, even as he; and ere long she would leave the world for ever, and her beauty become only a memory in song.

This doom she chose, forsaking the Blessed Realm, and putting aside all claim to kinship with those that dwell there; that thus whatever grief might lie in wait, the fates of Beren and Lúthien might be joined, and their paths lead together beyond the confines of the world. So it was that alone of the Eldalië she has died indeed, and left the world long ago. Yet in her choice the Two Kindreds have been joined; and she is the forerunner of many in whom the Eldar see yet, thought all the world is changed, the likeness of Lúthien the beloved, whom they have lost.

Chapter 20 Of the Fifth Battle: Nirnaeth Arnoediad

It is said that Beren and Lúthien returned to the northern lands of Middle-earth, and dwelt together for a time as living man and woman; and they took up again their mortal form in Doriath. Those that saw them were both glad and fearful; and Lúthien went to Menegroth and healed the winter of Thingol with the touch of her hand. But Melian looked in her eyes and read the doom that was written there, and turned away; for she knew that a parting beyond the end of the world had come between them, and no grief of loss has been heavier than the grief of Melian the Maia in that hour. Then Beren and Lúthien went forth alone, fearing neither thirst nor hunger; and they passed beyond the River Gelion into Ossiriand, and dwelt there in Tol Galen the green isle, in the midst of Adurant, until all tidings of them ceased. The Eldar afterwards called that country Dor Firn-i-Guinar, the Land of the Dead that Live; and there was born Dior Aranel the beautiful, who was after known as Dior Eluchíl, which is Thingol's Heir. No mortal man spoke ever again with Beren son of Barahir; and none saw Beren or Lúthien leave the world, or marked where at last their bodies lay.

In those days Maedhros son of Fëanor lifted up his heart, perceiving that Morgoth was not unassailable; for the deeds of Beren and Lúthien were sung in many songs throughout Beleriand. Yet Morgoth would destroy them all, one by one, if they could not again unite, and make new league and common council; and he began those counsels for the raising of the fortunes of the Eldar that are called the Union of Maedhros.

Yet the oath of Fëanor and the evil deeds that it had wrought did injury to the design of Maedhros, and he had less aid than should have been. Orodreth would not march forth at the word of any son of Fëanor, because of the deeds of Celegorm and Curufin; and the Elves of Nargothrond trusted still to defend their hidden stronghold by secrecy and stealth. Thence came only a small company, following Gwindor son of Guilin, a very valiant prince; and against the will of Orodreth he went to the northern war, because he grieved for the loss of Gelmir his brother in the Dagor Bragollach. They took the badge of the house of Fingolfin, and marched beneath the banners of Fingon; and they came never back, save one.

From Doriath came little help. For Maedhros and his brothers, being constrained by their oath, had before sent to Thingol and reminded him with haughty words of their claim, summoning him to yield the Silmaril, or become their enemy. Melian counselled him to surrender it; but the words of the sons of Fëanor were proud and threatening, and Thingol was filled with anger, thinking of the anguish of Lúthien and the blood of Beren whereby the jewel had been won, despite the malice of Celegorm and Curufin. And every day that he looked upon the Silmaril the more he desired to keep it for ever; for such was its power. Therefore he sent back the messengers with scornful words. Maedhros made no answer, for he had now begun to devise the league and union of the Elves; but Celegorm and Curufin vowed openly to slay Thingol and destroy his people, if they came victorious from war, and the jewel were not surrendered of free will. Then Thingol fortified the marches of his realm, and went not to war, nor any out of Doriath save Mablung and Beleg, who were unwilling to have no part in these great deeds. To them Thingol gave leave to go, so long as they served not the sons of Fëanor; and they joined themselves to the host of Fingon.