Alternate History for any D&D setting, with the main premise being that supernatural beings can't cross salt
Setup: Due to a meteor impact in ancient history, the entire planet has a thin field of salty space dust. Due to this field, outsiders can only enter this world by "riding" a shooting star that punches a hole in the field temporarily (usually followed by a small horde of lesser evils before the field closes up again).
In this world, paladin's are elite teams trained to track and hunt down evil, which are super obvious on divine radar I'm a world with no inherently evil beings. They see a star fall, they hunt for whatever got in, rinse and repeat.
- campaign starts here, with the players being paladins (in this case paladin is a title, not a class), getting adventures handed to them as orders come in to wipe out evil as it arrives and retrieving the evil artifacts left behind (meteor fragments with severe taint)
Event: A meteor shower causes an invasion of apocalyptic proportions. The support network of the order of paladins is crippled, and the players are now isolated. The final message from the order was a call for aid, followed by the sender dying horribly.
- the players now need to find a way back to whatever is left of the order in a newly hostile setting where hell is literally let loose on the world and meteor fragments are going full horcrux on people, causing evil to grow in the hearts of mortals.
Event: Upon reaching the headquarters, they find a bloodbath and the real threat: a containment breach. Every evil artifact collected over millennia has been scattered to the ends of the Earth. The goal shifts from "get back to base and regroup before containing a few dozen fragments" to "Cry in a corner when your workload increases to a few HUNDRED fragments and you find out you have no backup"
In case anyone notices...yeah, it's a medieval fantasy version of the SCP universe.