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The title was created due to the two brothers' spheres of influence over the spirit. Though often called by the name of dwelling places, the true names of the brothers were Námo, the elder, and Irmo, the younger.
Irmo, who resided in Lórien, was the master of visions and dreams. His gardens were the fairest in the world and filled with many spirits. Often the Valar would come to his gardens to rest and find ease from the burdens of Arda.
Námo, who resided in Mandos, had power over the fëar of Elves and Men who gathered in his halls after death, where the fëar of the Elves abode while those of Men soon left the Circles of the World. Námo delivered judgement on Manwë's bidding in serious matters (such as the Doom of the Noldor) but also judged over the fate of the elvish fëar.
Nienna is the sister of the Fëanturi, who mourned Melkor's marring of Arda. She resided west of West, near the halls of Mandos, which she often entered to bring strength and wisdom to the fëar there. However, she is never explicitly mentioned as a member of the Fëanturi.
Nienna and Mandos are counted among the Aratar - the eight greatest of the Valar - while their brother Irmo is not.
Other versions of the legendarium
The word produces the Qenya names Vefantur (the Fantur of Death, Mandos) and Olofantur (the Fantur of Dreams, Lórien).
- J.R.R. Tolkien, Christopher Tolkien (ed.), The Silmarillion, "Valaquenta: Of the Valar"
- J.R.R. Tolkien, Christopher Tolkien (ed.), The Silmarillion, "Appendix: Elements in Quenya and Sindarin Names", entries fëa, tur
- J.R.R. Tolkien, Christopher Tolkien (ed.), The Lost Road and Other Writings, Part Three: "The Etymologies", p. 387 (entry SPAN-)
- J.R.R. Tolkien, "Addenda and Corrigenda to the Etymologies — Part Two" (edited by Carl F. Hostetter and Patrick H. Wynne), in Vinyar Tengwar, Number 46, July 2004, p. 15
- J.R.R. Tolkien, Christopher Tolkien (ed.), The Book of Lost Tales Part One, Appendix: Names in the Lost Tales – Part I, entry "Fanturi"
- J.R.R. Tolkien, Christopher Tolkien (ed.), The Book of Lost Tales Part One, "III. The Coming of the Valar and the Building of Valinor", p. 66