The Shadow King's Acquaintance

From a failed possession to a symbiotic relationship, the shadow that attempted to possess you had failed it's ritual and ended up being partially left behind within your body. Over time you must build a bond with the shadow as it needs your life force to survive and you need it's skills to overcome the world that awaits. Additionally the shadow can choose to change your saving throw to a STR save if the result would end in you hitting 0 HP but only once per short rest.