I found the passage you were referring to:
Quote:
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In Eregion Sauron posed as an emissary of the Valar, sent by them to Middle-earth ("thus anticipating the Istari") or ordered by them to remain there to give aid to the Elves.
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I hadn't recalled that particular bit, and your thoughts on the subject help to throw some light on part of Sauron's reasoning behind his posing as Annatar.