Among the most depraved elite gourmets, unscrupulous wizards and debased wretches of society there is a fierce demand for a secret forbidden mushroom, one that only grows on the bodies of the undead. They're supposedly an unmatched delicacy, and rumored to let people see beyond the veil if prepared just so. While selling or possessing them is a crime, the law was willing to turn a blind eye because these fungi were so vanishingly rare that they were practically never seen.
Someone has begun to flood the market with them, in amounts that would require thousands of ghouls to produce and farm. They must be found and destroyed. Get to work.