Mandos was otherwise known as Námo or the Doomsman of the Valar.

He was the older brother of Lórien (Irmo) and Nienna. His mate was Vairë.

Together with his brother Irmo they were called the Fëanturi, or the "Masters of Spirits'.

Mandos dwelt in the Halls of Mandos where he called the spirits of the dead to him so that they could tarry for a time in his halls. There the spirits would find healing and comfort in repenting for any wrongdoing during their lives. After a time, the Elves could be reincarnated, but only Mandos, Ilúvatar, and possibly Manwë knew what the fate of Men might be.

Mandos appeared once as a shadowy figure and he told to the Noldor the Doom of the Noldor. He was also only once moved to pity by the song of Lúthien, after which he asked Manwë to spare Beren the mortal man whom she loved.
Encyclopedia entry originally written by atalante_star