[ainur]

I remember reading that, and I think at first I thought it refered to Fangorn and Lothlorien, neither of which would be a safe place for a Nazgul. It may, however, refer to Amon Lhaw and Amon Hen, the hills of Hearing and Seeing. There seems to be some kind of power in these places, aside from their vantage point overlooking Gondor's northern border on that side of the Misty Mountains. Frodo certainly sees visions while sitting on the Seat of Amon Hen, visions that may or may not be attributed to the fact he is wearing the Ring at the time. It had certainly never reacted like that any other time he put it on. What effect the power of this place may have had on the Nazgul is anybody's guess, if such power even existed there. It's possible Tolkien may have mentioned it somewhere, but I posess neither the letters nor any of the Histories of Middle Earth, so I cannot check on it. Perhaps, someone better-read in Tolkien can do so for you? I'm curious as well.