The Shin-keth are gnolls born into shadow. They run far larger than common gnolls and far stronger, and a grown one can step into a patch of darkness and simply not be there anymore. The trait breeds true. It is a lineage, not a working laid on a single animal, and a Shin-keth whelp comes into the world already half in the dark its parents hunt from.
What they carry is a Vulus affinity, an inborn pull toward the darkness layer that lets them thicken the shadow around them and pour themselves into it. That is shadow worked as a substance, the same stuff a dark-shaper pools and shapes, not a hole cut in the light and nothing to do with the dead. The gift sits over an ordinary living beast. A Shin-keth eats, breeds, and ages, and when one is killed it dies the plain death of any animal, its three strands loosed to the roads every dying thing walks, and the shadow-gift ends with the body that carried it.