Questioner

If someone—Vasher says that Nightblood would kill him, is that just because he has this one deific Breath? Would it kill an ordinary person, like a drab?
Brandon Sanderson

It would suck the Breath from anybody, and if they were unable to feed it he would feed on their soul.
Questioner

So they would die?
Brandon Sanderson

Yes. Anyone wielding Nightblood, he will suck their soul. If for too long, he will eventually—if you draw him—will suck your soul.