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If Nienna were to be touched by corruption it would not effect her disposition at first because she is so pure of heart and holds the powers of purification —
Instead her physical attributes would change first. Most notably I believe she would lose her eyesight. For Nienna loves her task of observing Eru’s children and though she may still hear their songs she would be unable to keep from sorrowing of her own loss, at least for a short time.