I have always been of the opinion that if Wakboth is the moral evil of the mundane world within time and trapped there, then the Wakboth you meet on a heroquest is formed from your own inner evil, that little shade of darkness at the heart of every mortal. Or, he can be formed by the evils of opposing forces on the quest. In either case, it is the mortal heroquester that brings the fuel that becomes the devil with them.
This is why some can call the Red Goddess, once mortal, the devil, while others call Argrath the same thing. Every mortal can be the devil, if given the opportunity.