Tricky... I'm not sure you could do it with a sword.
You could make a weapon-quality material that is just absurdly good in general, something as dense as slade but sharper than adamantine, for instance, and that would make it quite easy to one-hit-kill a demon or any other creature, but I suspect you're wanting something that would one-shot a particular creature instead of just a general creature.
The other option wouldn't work with melee weapons, and would be difficult to apply to demons, but would work with ranged weapons. You'd make a metal which melted at above room temperature but below body temperature and apply a nasty syndrome, like rapid full-body necrosis, massive bleeding, total paralysis, and so on, to it. If I remember correctly, when a weapon made out of the material gets stuck into a creature, it then gets warmed to its melting point by their body heat, and the molten metal then causes the syndromes. You'd have to list every demon under the syndrome's affected creatures (tricky, but not impossible), and it would really only work with ammunition since they're the only things that get stuck in long enough to melt, since shots only get stuck in instead of breaking occasionally it wouldn't work out to a one-hit-kill every time, it would have a somewhat delayed effect between the working shot and the syndrome actually taking effect, and it wouldn't work on inorganic demons, but at least you'd have demon-slaying ammunition.