In Death Masks, Harry Dresden describes it as congealed antilife, "a chip would make nuclear waste look like second hand smoke." Not from our reality, it's from Outside and sensitive to applied will.
In reaction to a bug placed into the box with it, it vaguely formed an inky cloud that floated up out of the box about the size of a baby's head. It had dozens of shadow tendrils that held the scorpion, violet flickers of flame burned the shell till it crumbled to dust. According to Thomas Raith, it looks like someone spray-painted a soap bubble and gave it tentacles.
In the series
For the duel, the ball that the Archive has formed has an enchantment to bind the mordite particles in place. It's sensitive to applied will. The duelists face each other with the mordite ball between then and will it to their opponent. The duel ends when one is devoured by it.
In Turn Coat, Samuel Peabody smashes an ink pot on the floor releasing a mistfiend laced in mordite, killing and wounding many of those attending Donald Morgan's trial with frightening smoky tendrils that could slice a human in half in a blink. The Senior Council, all of them, use applied will to contain it. Peabody infused it with mordite.