Board Thread:Skyrim/@comment-14027812-20130807032958/@comment-5351445-20140805173741

Perhaps Arniel somewhat bound himself to that soul gem when he struck it with Keening, and once he hit it hard enough it absorbed his soul like normal. However it went wrong because he has a black soul, and the warped soul gem can only fit grand souls or lesser. Therefore Arniel's soul was simply going to slip away to the afterlife, or someplace else entirely. Sensing his demise Arniel quickly latched himself on the closest living thing he could find, the Dragonborn. Due to his soul not being entirely contact from part of it slipping away he is only a shadow of his formal self, a shade if you will. Due to the newfound connection between the two, you can now summon Arniel's shade. Think Harry Potter. Voldemort cast the killing curse on him but his mother's love protected him. So the spell rebound and hit Voldemort. Voldemort then latched himself on the closest living thing, Harry. And by doing so, created a Horcrux he never wanted to make.