In their first year of schooling they had both been given versions of a thought experiment: a vampire goes skiing through a wooded slope, and accidentally gets impaled through the chest with a tree branch. Do they die?
Of course not - the wood by itself was almost harmless, even through the heart. Belief was the mallet that would drive the stake home, the light that would make the holy icon glow.
When Karet had first met the vogelfrei in Queen's Park in Toronto he hadn't bothered with conversation, trying to knock the target out from behind - not because he was a simple thug, but because he could hit the vogelfrei in the head as hard as he wanted and it would be merely incapacitating.
Reviewing the details of the Montreal attack, Karet didn't need to be told that the bouncer Leerman would be back on the job after several weeks. The mute pygmy Oliver would be hissing and annoying patrons again. Five years ago the vogelfrei had answered Karet's swing to the back of his head with a greater physical attack, and the graze to Karet's heart had required weeks for full recovery, but today he had not a single scar. The vogelfrei had destroyed Karet's entire left arm and pulped half of his chest with his wasteful physical strength, all for naught.
Tzaraa had gotten more than a physical hit - at the moment of the knee strike to her skull the vogelfrei must have fantasized about some girl who had frustrated him in his youth, and there had been an extra touch of something else, the vague thing called 'magic'. The dark magic that might allow a bullet in a shooting spree to find a target at an improbable distance, bypassing many obstacles and adults to strike a child. That was about as much 'dark magic' as most living humans could manifest before death - but after death so much more was possible.
For an undead to permanently kill another they needed to borrow that lethal human belief. It could be stored in certain weapons, like the falx that Zvera had swung against the vogelfrei in Toronto. Such a true weapon would be non-discriminant against which fiend of the night it claimed.
In his unschooled naiveté the vogelfrei had maximized the effectiveness of his chemical attacks with the air fresheners dispensing extracts from garlic, maple syrup, holy basil and roses along with coloidal silver, but he was really maximizing the faint power those substances had based on the ambient but faded belief of surrounding humans, with his own knowledge adding nothing. Personal, focused belief was so much more potent: if a human had tried the same thing it wouldn't have just caused an allergic reaction and general sluggishness.
Those same concoctions would have literally and permanently dissolved the bar's patrons like a powerful acid.