We already mentioned the bodies comment hinting that she's Death, but I think I thought of another one. When she was talking about how that spell and magic works in particular, she ultimately said "Magick, as it does, took the path of least resistance, ferried the closest, most appropriate candidate down. Or up." She totally came from hell, that's why she went up through the ground and looked like she was dead coming back to life. Had a witch from Earth been summoned, she would've arrived from the sky and not from under the Road. When they went on the Road, they went underground. She also had that demon voice.