Nullspice
Description: derived from the umbel of the root vegetable Dacus carota purgiato, a form of supernaturally-infused carrot native to the area around what used to be the Aral Sea. The umbel is collected, dried, and carefully powered. It tastes (pleasantly) somewhat of wintergreen, and promotes a sense of refreshment and cleansing when ingested. Nullspice is mundanely nontoxic, in regular doses; see below for supernatural complications.
It’s called that because it’s sort of the opposite of allspice, you see. Not entirely, of course: if nullspice and allspice are combined you won’t lose the East Coast in a sudden flash of light. But there’s a definite counter-reagent quality to the stuff.
What’s that about a counter-reagent? Well, as all men know ‘pumpkin spice’-flavored products are distributed (according to strict alchemical and ethical witchery precepts) among the general population every year to protect them against inimical magical influences. And it works! Enough people get dosed to provide herd immunity for the rest, and the incidents involving those who won’t partake of the spice can usually be handled on a case by case basis.
However, sometimes somebody needs to be quickly cleansed of the spice, usually to go undercover as a cultist or Dark Ritualist, or else to serve as bait for an operation. For that, they need a tincture of Nullspice; it’s a dandy way to cleanse all active and lingering apotropaic effects from a person’s aura, to the point where it’s impossible to tell if there was ever any protective alchemy placed on the person in the first place. Heck, Dark Ritualists themselves take regular doses of Nullspice. They have their own ways to ward off evil magic, and Nullspice keeps their system clean of substances that might otherwise adversely affect their access to dark forces.
Unfortunately, Nullspice is also useful as a subtle poison against good magicians, assuming that the taste and effect can be masked; ingesting a dose makes someone essentially defenseless against curses, maledictions, and ill-meant sympathetic magic. The trouble is that it’s not difficult to acquire and trivially easy to mundanely transport Nullspice, as it’s usually more convenient to keep the stuff off the restricted substances lists. Which means that when somebody does get murdered while dosed with the stuff, it’s hard to use mundane law enforcement to figure out the crime and the culprit.
But then: that’s what freelance teams of investigators are for, right?