Not a virus, not a meteorite, but the abandonment of "god".
The world is "formatted" by some incomprehensible higher-dimensional force. The old physical laws are partially invalidated and replaced by new, random, unreasonable and weird rules.
Human settlements barely function by relying on specific rules, such as "no light at night" and "no calling others by their first names." Outside the settlement, there is a disordered area with chaotic rules, where humans will randomly distort into "different phases" - it may be a puddle of screaming water, or it may be a clock tower of flesh and blood that is always walking.
In such a world, survival is no longer about fighting monsters, but a battle of wits and courage against the underlying logic of the world.
