The black market of magical materials. Affordable only by the richest members of high society. The most common kind of material sold are curatives and curiosities. Before the war, this was mostly alchemical tools, like those used for transmutation of materials. Or cosmetic salves and potions to modify the body in some only arguably supernatural way.
With the war though comes a new kind of merchandise. Magical weapons, as well as materials to make them are now in high demand on both sides of the war. The older witches' covens around Europe are entirely opposed to making, and even more to selling, weapons and magical tools of destruction.
Despite no-one being willing to make them anymore. A particular militaristic cult has somehow managed to procure a large amount of magical weapons. And has been selling them to the highest bidder. Leading to a dangerous magical arms-race in the trenches of the Great War.
