here's some "fanfic":
Goblin Slayer and his "sister" were out hunting and trying to make ends meet, when all of a sudden, his "sister" disappeared. It turns out that his "sister" is a soul sucking spirit that sucks out a mortal's potential and consumes it. She was well fed, but even spirits have NEEDS, and she went on a date with a mischievous spirit. Well, he was a weak ^&* loser, and they ended up being captured by the victims they were trying to play their pranks on.
Without the evil leech sucking his soul, Goblin Slayer finally got the chance to embark on an adventure. While previously, he struggled to kill a rabbit just to earn food, over the course of a week, he became a powerful swordsman, earnt coin, suitored chickz, and started rolling in bank. Eventually, he advanced into Demon Slayer, learned fire breathing, and fought with and slaughtered all sorts of titanic monsters, for even more bank.
Meanwhile, the benevolent ruler of the town strived to keep those wicked spirits confined, so that they may no longer wreck havoc upon the world. The mischievous male spirit was too weak, and it got eaten by a stray spider. However, that $%^& soul sucking spirit was too well fed, and they were unable to do anything to it but to keep vigil over its prison. But as we all know, humans are lazy and careless creatures, and they f-ed up. Eventually, after months of searching(he wasn't even), Demon Slayer finally "found" his "sister", and she re-established her dominance over him. He proceeded to, uh, rescue her, but all his potential and power immediately left him, and he became Goblin Slayer once again. In his powerlessness, their attempt at flight resulted in them falling into some hole.
The End.