The One, the Father of All, who sang the world into being through the music of the Ainur.
Ilúvatar, also called Eru, “the One”, is the single creator of Tolkien’s legendarium. Nothing exists that he did not first conceive.
The name is Quenya for “Father of All”. He dwells beyond the world, in the Timeless Halls, and does not take a body or walk among his creations. The Silmarillion opens with him: before anything was made, he alone existed.
Ilúvatar first made the Ainur, the Holy Ones, from his thought. He gave each a theme and bade them sing together. Their music grew into a great harmony, and the Ainur understood a little more of his design as they played it.
Melkor, the mightiest of the Ainur, wove his own themes into the music, seeking to bend it to his will. The harmony wavered into discord. Ilúvatar answered with new themes that turned the discord into a deeper beauty, until Melkor’s rebellion became part of the song.
Ilúvatar then showed the Ainur a Vision of the world their music had described, and in it saw the coming of his Children. The Ainur loved what they saw. He spoke the word “Eä”, “Let these things Be”, and sent forth the Flame Imperishable to give the Vision real existence. Many Ainur entered the world to labour in it.
Elves and Men were not part of the Music. They came from Ilúvatar’s own theme, and the Ainur did not shape them. Hence they are the Children of Ilúvatar. Their fates differ. Elves are bound to the world as long as it lasts. Men are mortal, and when they die they leave the world. Ilúvatar called this the Gift of Men.
Ilúvatar seldom intervenes. He accepts Aulë’s act of creating the Dwarves and adopts them. At the end of the Second Age, when the Númenóreans attack the Undying Lands, he breaks the world, destroys Númenor, and removes Valinor from the reach of Men. Otherwise his will works through the Valar and through the unfolding of the Music.