With granted. The jack-o-lantern, being made of pumpkin, rots rather quickly when hordes of flies are attracted to the wafting scent of rotting tacos. The jack-o-lantern begins to rot, its once jolly face distorting into something far more terrible than the most hideous carved pumpkins. As a mass of maggots writhe within the pumpkin, they find themselves unable to escape it, as the pumpkin collapses in upon them and blocks their exit. The maggots fuse, creating a hive mind. They use their pumpkin shell as a body, and go to the nearest dumpster. Here they acquire more maggots, and further their power. With each dumpster the now intelligent pumpkin drags itself to, it grows a little smarter, a little stronger, and a little faster. Soon it's a small construct made out of trash. Then it begins assimilating small animals - squirrels, rats, dogs - and becomes capable of faster movement. It drags itself through the midnight streets until it comes upon a lone drunk.
Soon, it begins assimilating humans. Before long, it towers many feet high, weighs tons, and is ravaging a city, engulfing buildings within its rotten mass and using them to smash anything that opposes it. Humanity is wiped out.
I wish for better vision.