Answer:
I only exist when you are here. Where you never were I can never be.
What gets sharper the more you use it?
I have fangs and enjoy piecing holes with a single bite.
A dragon's tooth in a mortal's hand, I kill, I maim, I divide the land.
Die without me, never thank me. Walk right through me, never feel me. What am I?
The answer I give is yes, but what I mean is no. What was the question?