Bad Seeds
Description: a twisted and gnarled hard organic mass, about the size of a peach. It feels hard, fibrous, and vaguely itchy to the touch, and smells unpleasantly of rusted iron and mildewed hay. Consuming one is contraindicated. There is a mild enchantment on Bad Seeds that allow them to be ignored under normal circumstances, up to an including baggage checks at airports.
It’s a monster seed. Plant it in the right (read: evil, cursed, corrupted, hexed, desanctified, whatever floats your boat) ground, and a monster comes out. Sure, it will take about forty years or so for the process to complete, which is one reason why there aren’t more monsters around: but all it takes is one deranged lunatic to have successfully gotten away with seeding an area with Bad Seeds a couple of generations ago for the whole situation to blow up in somebody’s face today.