Black dust-like substance
The spores themselves are microscopic and only visible in large concentrations, and even then appear as nothing more than fine black dust. This black residue pervades the vaults, growing in the cracks, seams, and crevices in the walls, floors, and ceiling.
The midnight spores themselves are a psychotropic extraplanar substance of unknown origin. They are not a poison or a disease, and immunity to such provides no protection, nor can magic that neutralizes such remove the effects of the spores. The spores can be absorbed through the skin as well as inhaled, so simply holding one’s breath provides no protection from exposure. However, the particular strain of midnight spores cultivated by Urschlar is very specific in its spores’ effects—the spores only affect living humanoid creatures. The spores are harmless to all other life forms. The midnight spores cause paranoia in those exposed to them. The insanity inflicted by the spores is especially virulent, and takes effect immediately.