Summary: It rusts in a field, waiting.
It rusts in a field, waiting. It was once cutting-edge technology, an automated combine harvester able to fuel itself from agricultural waste. Now, it lies abandoned and dormant, but still alive. The vines and weeds slowly climb up the side, warily testing the slumbering titan. On one day, it awakens once more.
Perhaps it was a short circuit. Perhaps a vibration in the ground jostled something important. Perhaps an insect met its doom wandering into the biofuel cell's intake valve. Perhaps electromagnetic interference stirred it back to life. Perhaps it was a hacker, trying to reactivate the machine from a world away. Perhaps it was the spirit of some irate druid or forgotten deity, long denied sacrifice. Whatever the cause, the consequences would be clear.
At the other end of the field was a wedding reception. The field it once patrolled was long fallow, so the thresher focused in on the nearest sources of organic matter. The reason its kin were long decommissioned was they'd all independently rediscover that calorie-dense red meat was superior fuel than dried grass and corn husks. Even when their processors were reinitialized to factory settings, their efficiency settings always compelled them to eat the richest, densest energy sources nearby.
The original threshers were recalled and re-equipped with more selective biofuel cells, which only utilized waste plastics and vegetable matter. The one that sat rusting in the field, however, was not. Instead, it bore down on the stunned guests, first devouring the dog chained to a nearby post. Devouring canine entrails only whetted its appetite for the carnage to come.
They called it the Blood Baptism, on account of how little was left of the revellers. The savvier guests got in their cars and drove away, while the nimble ones fled into the nearby farmhouse. The thresher even tried chasing them in, but only bent some of its blades on the front steps. By the time the police arrived, the thresher was long gone.
They found it errantly wandering near the woods, devouring a wounded deer. The only thing that finally stopped it was an anti-materiel rifle fired from a helicopter. That terminated the thresher's movement, but the ground team only closed in after armed drones unloaded on it with explosives and armor-piercing ammunition. They left it where it fell.
It still rusts in the field, waiting.