One way to look at evil is that it is a pollution. And like any other form of pollution, it takes forever to go away. You can’t just kill a witch or a redcap and assume that’s the end of it. The stain of evil has a way of lingering…and given the right soil, it won't be the Great Pumpkin that rises out of the pumpkin patch.
Jack-o’-lanterns aren’t the toughest of monsters by any means (CR 1), but their surprisingly wide range of special abilities for such a low-powered creature means they could still put the scare into PCs, especially if they attack as a patch. And even a single jack could serve as a great antagonist in a 1st-level murder mystery or horror adventure, especially if the PCs are prevented by adverse circumstances from bringing all their forces to bear against the plant.
Adventurers are hired (or more likely pressured by family members in town) to clean out the cottage of “Greeny” Ygraine, a purported witch who recently passed away. This mostly means fending off the wolves and lynxes that have come sniffing after Greeny’s goats, and examining and disposing of some minor malevolent charms and books in her hut. But once they leave the cottage they are set upon by Greeny’s true legacy”: a deadly jack-o’-lantern. Plus, the unharvested crops surrounding Greeny’s cottage are tinder-dry…so killing the exploding jack-o-lantern might set whole acres on fire.
A black-hearted pixie hates the folk of Chelm Heath. After witnessing a jack-o’-lantern consume an unlucky brownie, the faerie gleefully scoops up the plant creature’s excrement in a clay pot and hides it amid the goods gathered for Chelm Heath’s harvest festival. The newborn jacks should sprout just as the festival begins, sewing chaos. Meanwhile, the brownie’s kin do a casting to find their lost cousin, and when the dice lead them to the ruined festival, they at first blame the innocent townsfolk and seek revenge.
At a New England boarding school, a group of young friends is suspected of setting a snare that nearly choked the school bully to death. Sent to weed the pumpkin patch that is the supposed scene of the crime, the students uncover the real culprit—a jack-o’-lantern. Assuming they escape its first attack, telling a teacher will only get them branded as liars, and brandishing sharp weapons will get them expelled. And this isn't a problem they can avoid—since their evening restriction lasts for two weeks, they’ll have to return night after night to the jack’s lair, until either it is dead…or they are.
—Pathfinder Bestiary 4 160
I should note that the Bestiary 4 artwork seems to show a sickle-wielding jack-o’-lantern…an Advanced specimen, maybe?
No radio show this week. Was up in Boston doing the Easter thing and seeing the Decemberists. Two nights in a row, in fact, because my friends are crazy.