He was responsible for the judgement of the Spirits of all Elven dead. He also had responsibility for pronouncing the dooms and judgments of Eru Ilúvatar under Manwë and would be later known to the Elves as Mandos after his sacred Halls of Mandos, over which he presided, and where Elves would go when slain.
Mandos was the brother of Irmo (Lórien) and Nienna in the mind of Eru Ilúvatar. He and his brother Lórien were the "Fëanturi", the Masters of Spirits. His wife was Vaire the Weaver.
When Melkor began marring Arda before the coming of the Elves, Tulkas arrived and wanted to make war swiftly. At the bidding of Manwë, Mandos pronounced the doom of the Firstborn. After the captivity of Melkor, Mandos had once again pronounced the Doom. Melkor spent three Ages in the duress of Mandos thereafter, guarded by his hound Gorgumoth in Lumbi. Mandos judged Fëanor to leave Tirion for twelve years after Fëanor's drawing of sword against Fingolfin. After the destruction of the Two Trees, Yavanna asked for the light of the Silmarils, which Fëanor denied and he shall be slain, the first in Aman; but Mandos had spoken and revealed that Finwë was the first, having been slain at the steps of Formenos by Melkor. When the Ñoldor revolted against the Valar, Mandos appeared before them and pronounced the Doom of the Ñoldor.
Finwë, the first to be slain in Aman, then approached Mandos in his halls and begged for his wife Miriel to return to life. In exchange Finwë could never be reincarnated like other elves, and had to remain in The Halls of Awaiting forever. Mandos granted Finwë his wish.
Only once was he moved to pity, 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 to Middle-earth to begin their second life there as mortal beings with no certitude of life.
When Eärendil arrived in Aman to beg for assistance from the Valar, Mandos considered Eärendil and Elwing's fate as Half-elven and gave them and their descendants a choice: either to be counted among Elves or Men.
