Answer:
What is the smallest bridge in the world?
Useful tool for who in darkness dwell. Within you, corrupting like a deadly spell.
Though easy to spot, when allowed to plume, It is hard to see, when held in a room.
I am whole but incomplete. I have no eyes, yet I see. You can see, and see right through me. My largest part is one fourth of what I once was.
A caribbean shape that makes ships disappear
You use it between your head and your toes, the more it works the thinner it grows.