I was curious to know why Nightwalkers do not have a metabolism that can be quickened by transmuters. They have fangs, do they eat? I can see maybe an elemental not having one, but this I am not so sure.

Sort of kicks Scion Muters in the jimmy...even more.

Dark colored air whirls around a solid structure. Hoarse breathing
surrounds the figure, calling forth each desperate gasp for unnatural
air with all its strength. Only fiery red eyes, hovering near the
upper middle of this form, mark the vicinity of a possible head.
Sharp claws, attached to nothing but a thick whirling mass, clench
and unclench in a pulsing rhythm.

There's additional information available to cabal members, but the short answer is that they don't have organs that a transmuter knows how to manipulate.

valguarnera@carrionfields.com

Yet the outer guard at the chasm can be quickened.