The Borrower is a restless spirit and primal force of death.  It feeds upon the remains of others, "borrowing" them and incorporating them into itself.  Some say it is the soul of one who passed too early and still has a mission to complete but none know for sure. 
 
It has limited intellect but unmatched strength, growing with each new corpse it feeds on.  Unless controlled by outside forces, it does not attack the living.  In this case, it is being controlled by Sebastius Nefar.