Is there any weapon, or type of magic spell, that can permanently kill demons without having to be in the twisting nether or fel saturated places?
Also, since many kinds of beings can also respawn if not killed in their realm, like elementals, are there weapons to permanently kill them as well, again without being in their realm?
There are not really, no. Also worth noting on most worlds Elementals just natively live on the mortal plane directly. Our elemental planes are an artificial creation.
They do, however, mean that it is possible to artificially bind a planar being to another plane and thus make them vulnerable to destruction there.
There are weapons that are particularly good at banishing demons, but not outright killing them, as far as I know.
The God Emperor of Mankind and his golden aura of holy wrath.
Oh in WoW…not sure maybe the weapon of a Guardian?
What you would need would be a weapon that can suck their souls as you kill them and prevent them from returning to the Twisting Nether.
There’s a few like that. The Blood DK Artifact, for example. Frostmourne could likely have done the same before it was broken and turned into the BotFP. Not to mention, DHs themselves are demon-killing machines, given they can suck up the souls of the demons they kill to become more powerful.
I mean, we were only able to kill Ragnaros within the Firelands. Same with Al’akir and his domain, so there is that possibility.
The further WoW goes, the less sense everything makes.
It’s pretty well known that the Old Gods were driving everything into chaos, elementals included.
So instead of just eradicating the Old Gods… they needed to imprison them, because… they were deep rooted in corruption?
Hey wait a second, you elementals… that exist everywhere(even other planes), you need to be locked up in prisons! You bad things you… oh yeah, everything is an elemental… oh, never mind, that an ignored plot point. Remember dark shaman, they did things? They raised molten giants… and made it so Thrall had nothing to call upon?
There is Frostmourne but that was forged into Twin Blades.
There is a quest in outland that details demon killing blades. https://wowpedia.fandom.com/wiki/Illidari-Bane_Shard_(Alliance)
After being shattered in the battle with Arthas on top of Icecrown.
Cutscene gun.