As the teifling cast her spell, her vision became blocked as if a fog had just rolled in. However this was no fog. The room was filled with so much death and despair that a natural occurring aura of necromancy was present. The aura was so thick it blocked her vision into the room, and she could no longer see the bodies or treasure. She knew from her background in magic that at a site of tragedy and horror, such magic could be created, and over time the magic only grows stronger. This aura was definitely powerful enough to create undead. The blood bath created by the gang's slaughtering of themselves in greed hundreds of years ago was most likely the cause of it.