After sinking many hours into Dark Souls, I've come to the realization that the story is actually very complex, even if it's hidden or muddied most of the time. Even the multiplayer aspects of the game have an in-world explanation which ties into the universe.
This makes me wonder about other things which never seemed to "fit" except as a game mechanic. Is there such an in-game, world-relevant explanation for the fog covering doors which the player must choose to "traverse the white light" through?
I always thought it was just a technique to prevent the player from leaving once a boss battle had begun, but now I'm thinking maybe there's some hidden intricacy to it that I've missed. This thought is enhanced by the fact that the fog does not always block the path to a boss; very often it simply leads to another area and immediately dissipates when the player crosses it.
Best Answer
No, there is no known story justification.
However I have some speculation:
I can't find the source for this, but IIRC the in story explanation for invasions, summoning, and miracle resonance is players interacting with time in 'interesting ways'.
If the above is accurate, then I speculate, one reason for the the fog barriers is a boundary across time/space where these interactions with time are occurring. In multiplayer non-boss fog gates reappear and forces you to interact with the invader or the boss.