Corruption

One day a stranger appeared in town. Cloaked, wearing strange clothes. He talked about his travels amongst the stars. Naturally, the locals paid no attention to him. They even resolved to ridicule. All, but one. A simple drunkard approached the stranger and offered him a beverage. With a smile on his face, he said: "Don't mind 'em, friend. I'd love to hear your stories."
So the stranger obliged him. Few unreal stories and drinks later, as the bar was closing, the stranger put his hand on the drunkard and told him: "Thank you. You shall be spared. This I promise you."
He smiled and walked out into the darkness of the night.
The drunkard just stood there for a few moments, still not being sure if he heard the stranger correctly. So he ordered one more drink and shortly after, he passed out in an alley next to the bar.
Few days have passed and it seemed the stranger had disappeared from the town. Everything was going back to normal. It was then, when the first strange occurrence happened at town's water treatment plant. A worker has gone missing. Next day, another. In a week, nine people vanished. The others didn't want to come to work, fearing for their lives. Investigation revealed that one of the security cameras caught the stranger approaching the plant a week ago. Exactly that night, which he spent talking with the drunkard at the bar.
They tried to question him, but to no avail.
Next few days, search parties, even armed volunteers entered the water treatment plant in search of the stranger. With no luck, after five days, they've given up. The plant was closed and town officials tired to come up with a solution.
In the next few days, more people went missing. It turned out, that they were all seen entering the water treatment plant of their own volition. No matter the precautions taken, more and more people disappeared into the bowels of the building.
Some townsfolk started to claim that a certain fleshy growth suddenly sprouted out of their water spouts at home. After this discovery, the plant was opened and a search party entered the premises. What they saw was hard to describe. From every orifice, from every crack ... grew this strange flashy pulsating growth. Upon closer examination, traces of human bones, trinkets and even jewelry were found inside it. Decision was made that the corruption must be purged. Damaging it physically had no effect. Fire didn't help either. And even then, still more people were breaking into the plant, never to be seen.
The town slowly emptied itself, the corruption grew. New bio-mechanical structures appeared, seemingly releasing strange vapor. Plants rotted off. Animals either died, or fled. The air was heavy and the entire town was covered with a blanket of thick brown mist.
As far as he can remember, this fate befell almost every single settlement he came across.
Perhaps the entire planet is gone and he is the last living human.
Perhaps it's all just a bad dream of a drunkard, sleeping off another night of heavy drinking in an alley next to the local bar.
Or perhaps the stranger kept his promise and he has been spared after all, for all eternity.