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?
A vacuum cleaner. An exorcism prayer. The still beating heart of a freshly killed unicorn. A smoke sword. Blue cheese. An act of selflessness. Powerful wind magic. Honestly it could be many many things, it depends on how much effort it needs to be and what naturally weaves into the rest of the story.
In LotR, evil is generally terrified of crossing water. So, if there's rivers nearby... Also, this is a terrible question to ask on a forum. Only you know what's available in your world and what would make sense or how a character would obtain it and why he would. Like @plothog suggested, we would all spew out random things but that's not really useful.
If it's made of smoke, how about counter it with something like ice magic? Once it's frozen, shatter it. If it's not physically there, then where is it? Where's this thing's physical location? Locate that and kill it. Kill it with its own reflection? Wind magic! Blow it apart If it's smoke, then it wouldn't be able to hold its shape and therefore its power in the face of a howling wind. An opposing gas that prevents it from reassembling. Personally, I like the vacuum cleaner idea above
Often, the key to what destroys a creature lies in how it is created or comes to be in the first place. Vampires are inherently unholy, so their destruction is tied to religion (crosses or holy water) or purity in nature (a take made of oak, or garlic, often associated with health, or the direct rays of sunlight). It's a bit hokey, in a way, but it harkens back to the notion of sympathetic magic, which is widely used in fantasy and science fiction. Kryptonite is lethal to Kryptonians - just about anything of their home planet is harmful to them, although other Kryptonians aren't , except for being innately aggressive. Ray Palmer is exposed to rays from a white dwarf star, and becomes the Atom, able to shrink at will to microscopic size and back again. Hopefully this will help you find your own answer,
Freeze and smash for temp. Idk ice spell or frosted blade. Maybe freeze and stab a specific place ie a heart forms to be stabbed with specialist weapon for fatal kill
