I theorize that A long time ago Salem and Oz were Siblings, Or maybe even past Lovers. But Salem got sick/hurt.. The first thing ever asked to Jen was How can I save Salems Life, the answer was infusing her cell with the cells of a Grim. The infusion of the Grim Cells turned Salem into what she is now.. She slowly become more destructive as time passed, The god brothers got upset with Oz for creating this abomination and forced him to live forever until she is stopped. The grim inside of Salem allows her to control Grim, and even create them with her own blood. This is of course all a theory.. I'm not sure of it, of course but. Man, I feel like that is were this is headed.