The Ringwraiths aren't the only beings that can use fear as a weapon. During
the confrontation with Sauron at the gates of Mordor, the Mouth of Sauron
reacted as if struck with blow although Aragorn did nothing but look at him. I
know that the situation was different, and the Mouth of Sauron was not a
Ringwraith but a mortal man, but I wouldn't be surprised if Aragorn had a
similiar effect on the
Nazgul.
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Although now long estranged, Man is not wholly lost nor wholly changed.
Dis-graced he may be, yet is not de-throned,
and keeps the rags of lordship once he owned:
Man, Sub-creator, the refracted Light
through whom is splintered from a single White
to many hues, and endlessly combined
in living shpes that move from mind to mind.
Though all the crannies of the world we filled
with Elves and Goblins, though we dared to build
Gods and their houses out of dark and light,
and sowed the seed of dragons--'twas our right
(used or misused). That right has not decayed:
We make still by the law in which we're made!