Another note here is that Nightblood can sense where Vasher is. This is because Nightblood has ingested and fed off Vasher’s Breaths in the past. When he does that, it connects him to someone. It’s also, by the way, one of the secrets as to why Vasher doesn’t get sick when holding Nightblood, even though he’s a good person. It’s not simply familiarity (though that is part of it). Nightblood has a built-in test. If he feeds off you and you survive, then you become somewhat immune to his powers.