Where Light and Void clash, chaos is born: the chaos of the Twisting Nether, birthplace of the Fel. This magic was literally birthed by annihilation when Light and Shadow came into terrible contact with one another. Fel magic is of entropy, of destruction, of the utter destruction of that which makes life possible. Void magic is the absence of Light, but Fel is born from its death.
Thus the Twisting Nether itself is not a shadow but a wound, and the first Demons were born from it. At the moment of its creation Fel magic made the Nether what it is. Demons are the creatures native to that place of entropic annihilation, born of the death of life itself. If you’ve ever seen a Soul Engine, you know exactly how this process works – the souls of the Legion’s victims are slowly destroyed over centuries to generate the Fel.