Judge of the Dead, chief advisor to Manwë Súlimo, Vala of Eru

Mandos is the usual name for the Vala originally called Námo (Judge). Námo was given this new name in honor of the Halls of Mandos, over which he presides, where Elves go after they are slain. His wife is Vairë the Weaver, and he is the brother of Lórien and Nienna in the mind of Eru Ilúvatar.

Mandos is described as being stern and dispassionate and never forgetting a thing. He was the Vala who cursed the Noldor leaving Aman, and counseled against allowing them to return (almost to the point of vindictiveness). But unlike Morgoth, his Dooms are not cruel or vindictive by his own design. They are simply the will of Eru, and he will not speak them unless he is commanded to do so by Manwë.

After a time, short or long, in the Halls of Mandos, elves are re-embodied or reincarnated. Elves who do not heed the call of the West gradually fade and diminish. The souls of men pass through his realm to another fate he knows not of.

Only once has he been moved to pity a mortal, when Lúthien sang of the grief she and her lover Beren had experienced in Beleriand. Then, with the Elder King's approval, he released them.

The event of Mandos being moved to pity by Lúthien singing of the grief she and her lover had been moved to in Beleriand, is similar to the legend of Orpheus and Eurydice, which partly inspired the story of Beren and Lúthien.

Written later in Tolkien's life, there arose the possibility that Glorfindel, was given second life after being slain by a balrog, and returned to Arda. It has been postulated that he was sent back to Middle-earth as a means to fight Sauron, just as the five Istari were.

It is known that other Elves were either re-embodied or reincarnated into new Hröa. This figured in Manwë's decision on marriage law when Finwë requested one - only those whose deceased spouse was tired of returning from the Halls of Mandos could remarry. (Notably, after Finwë passed to the Halls of Mandos, His first wife Míriel returned to life and entered the service of Vairë. So, like most divine laws in Eru's canon, it was immediately complicated if not outright broken, and no blame attended either party that we are cognizant of.)

It is known that Elves from the Blessed Isle visited Numenor regularly, though it is unknown how often they visited the Western shore of Middle Earth. Certainly traffic in the other direction continued, and the Fëanoreans crossed from Valinor in the First Age, via ships from the Blessed Isle, and over the ice.

Glorifindel may be the only known example of an elf being slain and directly sent back in (mostly) the same Hröa to Middle Earth, although Lúthien traveled physically in both directions, and Olorin the Maiar seems to have died and been near-instantly re-embodied in Arda in the Third Age. There is no record that other cases do or do not exist, nor of any prohibition against elves traveling in either direction.

Many elves traveled both ways in the War of Wrath with the Host of the Valar. Earendil traveled West with Elwing, and was set to sail in the sky as a guard. He notably returned to Middle Earth with his ship Vingilot to throw down the greatest dragon Ancalagon on the Mountainside.