Eru Ilúvatar is the supreme being in J.R.R. Tolkien's Middle-earth legendarium. He is introduced in The Silmarillion as the creator of all existence. In Tolkien's invented Elvish language Quenya, Eru means "The One", or "He that is Alone" and Ilúvatar signifies "Father of All". The names appear in Tolkien's work both in isolation and paired (Eru Ilúvatar).
Eru as Creator
Eru is the supreme being, God. Eru is transcendent, and completely outside of and beyond the world. He first created a group of angelic beings, called in Elvish the Ainur, and these holy spirits were co-actors in the creation of the universe through a holy music and chanting called the "Music of the Ainur", or Ainulindalë in Elvish.
Eru alone can create independent life or reality by giving it the Flame Imperishable. All beings not created directly by Eru, (e.g. Dwarves, Ents, Eagles), still need to be accepted by Eru to become more than mere puppets of their creator. Melkor desired the Flame Imperishable and long sought for it in vain, but he could only twist that which had already been given life.
Eru created alone the Elves and Men. This is why in The Silmarillion both races are called the Children of Ilúvatar. The race of the Dwarves was created by Aulë, and given sapience by Eru. Animals and plants were fashioned by Yavanna during the Music of the Ainur after the themes set out by Eru. The Eagles of Manwë were created from the thought of Manwë and Yavanna. Yavanna also created the Ents, who were given sapience by Eru. Melkor instilled some semblance of free will into his mockeries of Eru Ilúvatar's creations (Orcs and Trolls).
Eru's direct interventions
In the First Age, Eru created and awoke Elves as well as Men. In the Second Age, Eru buried King Ar-Pharazôn and his Army when they landed at Aman in S.A. 3319. He caused the Earth to take a round shape, drowned Númenor, and caused the Undying Lands to be taken "outside the spheres of the earth". When Gandalf died in the fight with the Balrog in The Fellowship of the Ring, it was beyond the power of the Valar to resurrect him; Ilúvatar himself intervened to send Gandalf back.
Discussing Frodo Baggins' failure to destroy the Ring, Tolkien indicates in Letter 192 that "the One" does intervene actively in the world, pointing to Gandalf's remark to Frodo that "Bilbo was meant to find the Ring, and not by its maker", and to the eventual destruction of the Ring even though Frodo himself fails to complete the task.
Tolkien on Eru
In 1954 Tolkien discussed his role as "sub-creator" in a draft of a letter to Peter Hastings, manager of the Newman Bookshop (a Catholic bookshop in Oxford). In the draft, Tolkien, himself a devout Roman Catholic, defended the portrayal of God (Eru) in his writing as an exploration of the infinite "potential variety" of God: that it need not conform to the reality of our world so long as it does not misrepresent the essential nature of the divine.
God has not used that device in any of the creations of which we have knowledge, and it seems to me to be stepping beyond the position of a sub-creator to produce it as an actual working thing, because a sub-creator, when dealing with the relations between creator and created, should use those channels which he knows the creator to have used already.
Tolkien's reply explains his view of the relation of (divine) Creation to (human) sub-creation:
We differ entirely about the nature of the relation of sub-creation to Creation. I should have said that liberation "from the channels the creator is known to have used already" is the fundamental function of "sub-creation", a tribute to the infinity of His potential variety [...] I am not a metaphysician; but I should have thought it a curious metaphysic — there is not one but many, indeed potentially innumerable ones — that declared the channels known (in such a finite corner as we have any inkling of) to have been used, are the only possible ones, or efficacious, or possibly acceptable to and by Him!
As for Tom Bombadil, I really do think you are being too serious, besides missing the point. [...] You rather remind me of a Protestant relation who to me objected to the (modern) Catholic habit of calling priests Father, because the name father belonged only to the First Person.
Inspiration and development
In earlier versions of the legendarium, the name Ilúvatar meant "Father for Always" (in The Book of Lost Tales, published as the first two volumes of The History of Middle-earth), then "Sky-father", but these etymologies were dropped in favour of the newer meaning in later revisions. Ilúvatar was also the only name of God used in earlier versions — the name Eru first appeared in "The Annals of Aman", published in Morgoth's Ring, the tenth volume of The History of Middle-earth.
- The Silmarillion, p. 329; the root er means "one" or "alone" (p. 358)
- The Silmarillion, p. 336; from ilúvë ("all, the whole", p. 360) and atar ("father", p. 356).
- Tolkien, J. R. R. (1993), Christopher Tolkien, ed., Morgoth's Ring, Boston: Houghton Mifflin, ISBN 0-395-68092-1
- In Letters, #156, pp 202–3, Tolkien clearly implies that the 'Authority' that sent Gandalf back was above the Valar (who are bound by Arda's space and time, while Gandalf went beyond time).
- Carpenter, Humphrey, ed. (1981), The Letters of J. R. R. Tolkien, Boston: Houghton Mifflin, #192, ISBN 0-395-31555-7
- Carpenter, Humphrey, ed. (1981), The Letters of J. R. R. Tolkien, Boston: Houghton Mifflin, #153, ISBN 0-395-31555-7
- Eru Ilúvatar at the Lord of the Rings Wiki