Silmarillion Chapter 8: Of The Darkening of Valinor
But now on the mountain-top dark Ungoliant lay; and she made a ladder of woven ropes and cast it down, and Melkor climbed upon it and came to that high place, and stood beside her, looking down upon the Guarded Realm.
…Then Melkor laughed aloud, and leapt swiftly, and leapt swiftly down the western slopes; and Ungoliant was at his side, and her darkness covered them.
A lot of the supernatural monsters in Tolkien’s mythology are deliberately left vague. In general, this is to leave it up to the imagination, but in the case of Ungoliant, I think it’s more interesting to depict her as a spider-shaped void than a detailed monster. She is, after all, more of an elemental, the embodiment of hunger. Similar to what I’ve done with the Valar, I think Ungoliant’s form is more of a vague facade she wears to interact with the world.