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It is thought that she may have been one of the Maiar whom Melkor corrupted long ago, but she is not listed among the Ainur. It is also said, arguably separately, that she came from the darkness above the skies of Arda, and this reference has led some to believe she may be an incarnation of darkness or emptiness itself, as this seems very consistent with the rest of her character.
Ungoliant aided Melkor in his attack on the Two Trees of Valinor; she drained the Trees of their sap and poisoned them after Melkor wounded them, and she drained dry the Wells of Varda, so that nothing remained of the Light that was before Sun or Moon, except in the Silmarils of Fëanor. She and Melkor fled to Middle-earth to escape justice at the hands of the Valar.
Melkor had promised to reward her, "Yea, with both hands", and after their flight from Valinor, the Dark Lord indeed gave her many gems of the Ñoldor, but withheld the Silmarils in his right hand. He refused to give them to the Great Spider to be devoured, for he desired them greatly, and Ungoliant would have slain Melkor in the ensuing battle had not the Balrogs come and saved their master, causing Ungoliant to flee to the Ered Gorgoroth in Beleriand.
While there, she had many offspring, including Shelob, as well as various other creatures infesting the Ered Gorgoroth, which came to be a place of horror. The dates of her existence are not precisely known; in The Silmarillion, Quenta Silmarillion, Chapter 9, Of the Flight of the Noldor, we are told that she "went whither she would into the forgotten south of the world" shortly before the First Age, and that "some have said that she ended long ago, when in her uttermost famine she devoured herself at last." In contrast, a rough sketch of Eärendil's voyages by Tolkien suggests that he slew Ungoliant in the south.
The name is pronounced [uŋˈɡoljant]. In the form Ungoliant the name is technically Sindarin, but is a direct loan from Quenya Ungweliantë (ungwë + liantë: "dark spider"; pron. [uŋʷˌɡʷeliˈante]); the strictly Sindarin form being Delduthling "dark terror spider."
She is also called "gloomweaver," Virilomë [ˌviriˈlome]) (Wirilomë in Vanyarin) which becomes Gwerlum ([ɡʷerlum]) in Sindarin.