A devil defeated by the lord of a evil lair is sealed away for many years, and is finally released through the power of a captive young girl.
As a defenseless human girl, there's no way she can escape the powerful hordes of the lord by herself.
So, in exchange for your soul, you make a contract with that devil yourself to help her escape.
But you'll need more than one soul to take on the lord of the lair; you'll need the souls of all the creatures you fell along the way.