That Which Stalks

What is it?
This thing which stalks me,
Seeks to invade the mind and still the heart.
I cannot escape.
It takes hold, and drags, and drags....
I fall.
Darkness engulfs me.
It lives and breathes within me.
It taints me with foul, clogging breath.
I fling my arms, I scream, I cry,
I try my utmost to escape.
I beat up, up, up
I can see the light!
But it comes again, it snatches light from me,
It kills liberty
And enslaves me.