Hello, I've arrived at an impass. In my story, a Ringwraith like creature of immense power happens upon the protagonists: It's basically a black mist that, when it confronts people, takes on a vaguely humanoid shape with a smoke sword. It's not actually physically present there, but it can hurt people physically and can be killed by x (even though after that the mist can come back/reassemble). I can't figure out what can temporarily kill them (x) and what can kill them forever. Any suggestions?