@Foxtrot94 Just because Arius believed he will become immortal through Argosax, doesn't mean he actually would. People are prepared to believe all sorts of bullshit if it means a lot to them and it's presented in a plausible way.
And that doesn't necessarily mean he was wrong either. Even though we actually don't have proofs that demons are actually immortal, that's still an hint that they are, while there's nothing that indicates they are not. Did we actually see a demon dying by ageing? Nope.
So, we don't have proofs on both sides. But at least we have a hint that demons can be immortal, while we have none that they can't. No reasons for me to believe that they're not immortal, then.