Silmarillion: Chapter 8 : Of the Darkening of Valinor
..." and there in Avathar, secret and unknown, Ungoliant had made her abode. The Eldar knew not whence she came; but some have said that in ages long before she descended from the darkness that lies about Arda, when Melkor first looked down in envy upon the Kingdom of Manwë, and that in the beginning she was one of those that he corrupted to his service. But she had disowned her Master, desiring to be mistress of her own lust, taking all things to herself to feed her emptiness; and she fled to the south, escaping the assaults of the Valar and the hunters of Oromë, for their vigilance had ever been to the north, and the south was long unheeded. Thence she had crept towards the light of the Blessed Realm; for she hungered for light and hated it...."
"in the beginning she was one of those that he corrupted to his service"
Il me semble qu'il est clairement sous-entendu, un des Ainur qu'il corromput alors (tels Sauron ou les Balrogs)...