Answer:
The person who makes it has no need for it. The person who purchases it does not use it. The person who does use it does not know he or she is. What is it?
Useful tool for who in darkness dwell. Within you, corrupting like a deadly spell.
I'm the source of all emotion, but I'm caged in a white prison.
It can pierce the best armor and make swords crumble with a rub, yet for all its power It can't harm a wooden club.
Gets rid of bad ones, short and tall. Tightens when used, one size fits all.
What never gets any wetter no matter how hard it rains?