There is no such thing as invulnerability. And even if someone can't be killed, he can be sealed away.If the shadow dude is invulnerable AND killed the head of state why not just kill the saint as well? Seems he's so powerful the demon army doesn't even need to fuck with anyone else.
He can teleport in, kill anyone he wants, takes anything he wants, and no one can stop him cause he's immune to death?
Why isn't he killing everyone in this series instead of having them fuck around with "challenges"?