Mandos - The Halls of Waiting
The Fëanturi, masters of spirits, are brethren, and they are called most often Mandos and Lórien. Yet these are rightly the names of the places of their dwelling, and their true names are Námo and Irmo.
Námo the elder dwells in Mandos, which is westward in Valinor. He is the keeper of the Houses of the Dead, and the summoner of the spirits of the slain.
Vairë the Weaver is his spouse, who weaves all things that have ever been in Time into her storied webs, and the halls of Mandos that ever widen as they ages pass are clothed with them.
... Nienna, sister of the Fëanturi. She dwells alone. She is aquainted with grief, and mourns for every wound that Arda has suffered in the marring of Melkor. ... But she does not weep for herself; and those who hearken to her learn pity, and endurance in hope. ... She goes rather to the halls of Mandos, which are near her own; and all those who wait in Mandos cry to her, for she brings strength to the spirit and turns sorrow to wisdom.
It is one with this gift [by Ilúvatar] of freedom that the children of Men dwell only a short space in the world alive, and are not bound to it, and depart soon whither the Elves know not. Whereas the Elves remain until the end of days.... For the Elves die not till the world dies, unless they are slain or waste in grief (and to both these seeming deaths they are subject); neither does age subdue their strength ... and dying they are gathered into the halls of Mandos in Valinor, whence they may in time return. But the sons of Men die indeed, and leave the world.... Death is their fate, the gift of Ilúvatar, which as Time wears even the Powers shall envy. ... Yet of old the Valar declared to the Elves in Valinor that Men shall join in the Second Music of the Ainur; whereas Ilúvatar has not revealed what he purposes for the Elves after the World's end, and Melkor has not discovered it.
Silmarillion, "Of the Beginning of Days"
What may befall [Human] spirits after death the Elves know not. Some say that they too go to the halls of Mandos; but their place of waiting there is not that of the Elves, and Mandos under Ilúvatar alone save Manwë knows wither they go after the time of recollection in those silent halls beside the Outer Sea. None has ever come back from the mansions of the dead, save only Beren son of Barahir, whose hand had touched a Silmaril; but he never spoke afterward to mortal Men. The fate of Men after death, maybe, is not in the hands of the Valar, nor was all foretold in the Music of the Ainur..
Silmarillion, "Of Men"
[I *think* he means "of all beings lower than Ilúvatar, only Mandos and Manwë know ...".]
[Lúthien] set her arms around Beren, and kissed him, bidding him await her beyond the Western Sea....
The spirit of Beren at her bidding tarried in the halls of Mandos, unwilling to leave the world, until Lúthien came to say her last farewell upon the dim shores of the Outer Sea, whence Men that die set out never to return. But the spirit of Lúthien fell down into darkness, and at the last it fled, and her body lay like a flower that is suddenly cut off and lies for a while unwithered on the grass
... But Lúthien came to the halls of Mandos, where are the appointed places of the Eldalië, beyond the mansions of the West upon the confines of the world. There those that wait sit in the shadow of their thought. ... and she knelt before Mandos and sang to him ... and Mandos was moved to pity, who never before was so moved, nor has been since.
Therefore he summoned Beren, and even as Lúthien had spoken in the hour of his death they met again beyond the Western Sea. But Mandos had no power to withhold the spirits of Men that were dead within the confines of the world, after their time of waiting; nor should he change the fates of the Children of Ilúvatar.
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 ending 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, and subject to a second death, even as he; and ere long she would leave the world for ever.
Silmarillion, "Of Beren and Lúthien"
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 ever been heavier than the grief of Melian the Maia in that hour.
Silmarillion, "Of the Fifth Battle"
[to the rebelling Noldor:]
'... For though Eru appointed to you to not die in Eä, and no sickness may assail you, yet slain ye may be, and slain ye shall be: by weapon and by torment and by grief; and your houseless spirits shall come then to Mandos. There long shall ye abide and yearn for your bodies, and find little pity though all whom ye have slain should entreat for you. And those that endure in Middle-earth and come not to Mandos shall grow weary of the world as with a great burden, and shall wane, and become as shadows of regret before the younger race that cometh after.'
Silmarillion, "Of the Flight of the Noldor"
It is indeed unhappy,' said Míriel, 'and I would weep, if I were not so weary.' ... She went then to the gardens of Lórien and lay down to sleep; but though she seemed to sleep, her spirit indeed departed from her body, and passed in silence to the halls of Mandos.
Silmarillion, "Of Fëanor"
[Finwë] knew that he should not see [Elwë] again, unless it were in the halls of Mandos.
Silmarillion, "Of Eldamar"
[Fëanor's] likeness has never again appeared in Arda, nor has his spirit left the halls of Mandos.
Silmarillion, "Of the Return of the Noldor"
[on the accidental death of an Elf with whom he had recently quarreled:]
'I did not will it, but I do not mourn it,' said Túrin. 'May Mandos judge him justly; and if ever he return to the lands of the living, may he prove wiser.'
Unfinished Tales, "Narn i Hnîn Húrin"
Aforetimes it was held among the Elves in Middle-earth that dying the Dwarves returned to the earth and the stone of which they were made; yet this is not their own belief. For they say that Aulë the Maker, whom they call Mahal, cares for them, and gathers them to Mandos into halls set apart; and that he declared to their Fathers of old that Ilúvatar will hallow them and give them a place among the Children in the End. Then their part shall be to serve Aulë and to aid him in the remaking of Arda after the Last Battle. They say also that the Seven Fathers of the Dwarves return to live among their own kin and to bear again their ancient names: of whom Durin was the most renowned in after ages.
Silmarillion, "Of Aulë and Yavanna"
But when the Battle was ended ... the Valar drew Melkor back to Valinor, bound hand and foot, and blindfolded.... And he was cast into prison in the fastness of Mandos, whence none can escape, neither Vala, nor Elf, nor mortal Man. Vast and strong are those halls, and they were built in the west of the land of Aman. There was Melkor doomed to abide for three ages long, before his cause should be tried anew, or he should plead again for pardon.
Silmarillion, "Of the Coming of the Elves"
Going west from Middle-earth, you encounter the Western (or Inner) Sea.
Crossing that (perhaps with a stopover in Númenor), you come to the continent of Aman, which is the Far West and the Undying Lands.
Moving west across the continent, you pass the Pelóri Mountains, the plain of Valinor which contains the city of Valimar and the homes of the Ainur, and come to the shores of the Outer Sea. The halls of Mandos are on this farthest shore, looking out toward the Void.