The shadow is black smoke. The black smoke it shadow. It moves consciously with purpose. It wraps bones, somehow harmonises best with that specific material. The shadowsmoke is cold to touch but not like ice, more like a cold girlfriend’s hands which need warming; it is silky soft and sensual.
The smoke likes to pool but also it separates into whisps, that it is impossible to know how much of the stuff there is in any given pool, it’s volume is deceptive. A jar of it may wrap a whole cemetery of bones to create an army, each unit functioning independently while simultaneously the whole of them working together as one.
Shadowsmoke is a form of demon summoned by those who know how. It is silent and it hides.
There is a story, it can wrap bones of the living and compete with them for dominance of their own skeleton until they go insane, hollow, starving mindless husks gaunt and technically dead. Such zombies of legend are not exclusive to the Human species. Anything with a skeleton will do, if they can get in.
Some claim Shadowsmoke fears fire and sunlight, others say it withers from silver or water or prayer. We tried versions of this to find what works best; until we became bored and gave up on it.
The stuff slipped our minds and we realised only then, too late for our researches to bear substantial fruit, how it protects itself from such experimentation, from humans learning to master it completely, to control it by any means other than dark magic.
Of course you won’t believe in the stuff any more than you believe in magic. It’s out there though. Waiting.